Were William Butler Yeats alive, his passion for Celtic myth and legend would seize upon the story of Shergar.

Without even chatting with a bookie or barkeep, Yeats would arrive here in County Kildare and sense immediately the absence of the $13 million stallion who won the 1981 Epsom Derby by the greatest margin ever, walked off with the Irish Sweeps Derby and then was stolen from the Ballymany Stud farm by kidnapers 18 months ago. Yeats would know intuitively how the theft and probable killing of this magnificent beast violated the affections and pride of a nation uniquely attached to horses and the art of breeding.

The principal owner of Shergar was the Aga Khan, the spiritual leader of the Isma'ilis, an Islamic sect with 20 million members in Africa and the Middle East. The sect used to award the Aga Khan his weight in gold and precious gems every year. He owns houses all over the world and has a passion for Rolls-Royce motor cars. But for all the Aga Khan's wealth, negotiations between the kidnapers and the Shergar syndicate dissolved after only four days. Any subsequent calls to the police or the stud farm were fraudulent.

The police, the press and the people here are almost unanimously agreed that Shergar is dead.

A racehorse is too hard to hide, they say, too hard to keep healthy without the care of experts. The prevalent theory is that the kidnapers "put down" Shergar after negotiations reached an impasse. No body or lesser traces have been found and, though the police in the nearby town of Naas continue to investigate all leads, their efforts have dwindled.

But like Sweeney and Fergus and Cuchulain and the myriad mythic figures of Ireland, Shergar now has an everlasting life in the people's collective imagination, Yeats' spiritus mundi. There was and is no forgetting the incident. For months after the kidnaping, some Irish and British newspapers maintained full-time "Shergar bureaus." The police and the endless stream of reporters sought out any lead, no matter how preposterous.

Most speculate that the Irish Republican Army needed funds to continue its war against the British control of Northern Ireland and is responsible. But the IRA usually claims responsibility for its actions, no matter how violent. Nobody claimed responsibility for the disappearance of Shergar. Perhaps no one dared.

Even now, there are constant reminders of Shergar, a great beauty of a horse with a distinctive white blaze and white stockings. He stood at stud for just one season before the kidnaping, and this October four of his offspring will be offered for sale at the Newmarket auction in England. Shergar was extraordinarily fertile, putting 42 of his 44 mares into foal. He was expected to cover 55 mares in his second season at stud. To have a mare serviced by Shergar, the charge was $78,000, so the kidnaping cost the owners more than $4 million in fees for 1983 alone.

Shergar's offspring might inherit his capacity for speed and endurance; they surely are the living embodiments of his myth. Had Shergar lived as long as the legendary Northern Dancer, who is 23, he would have sired hundreds of foals. Irish racing has been stunted by his loss.

"Shergar was the greatest of animals," said Stan Cosgrove, the stallion's veterinarian and one of 34 partners in the 40-share syndicate. "Not huge. Not overwhelming at first sight. But just the proper muscular structure, great legs, fabulous bones. A perfect racer. The kidnaping, it's the first time such a thing has ever happened in this country. I never thought anyone would touch a horse in Ireland. Secretariat may have been a better racehorse, but I don't think Americans would react to his loss the way we Irish did with Shergar. Never in Ireland."

So Yeats, who wrote "At Galway Races" and had " . . . Horseman pass by!" etched on his limestone tombstone in the Drumcliff churchyard, might have uncovered the myth in the Shergar story. But he would never find the horse itself. No one has, and most here think no one ever will. THE KIDNAPING

The topography of Ireland is much like a shallow bowl. Limestone cliffs ring the lip of the dish and flatlands of peat and meadow lie in the middle. A half-hour's drive west of Dublin, a traveler sees one field after another of horses languidly craning their necks to nibble on juicy tips of grass. The grasses of County Kildare have the color and, to horses fortunate enough to live here, the sweetness of Granny Smiths. For centuries, horsemen have known that because Ireland's soil is rich with lime, horses bred here develop remarkably sturdy bones, providing the skeletal strength needed to support a thoroughbred's musculature.

The Aga Khan turned down bids as high as $35 million for Shergar from the Kentucky bloodstock industry. Instead, he preferred to keep the horse in Ireland and kept six of the 40 shares for himself. In recent years, the Irish tax code has encouraged the breeding industry to compete with the United States and Europe. The moist climate and the people's natural affinity for the animal make Ireland, County Kildare especially, a perfect horse farm.

Ballymany Stud, down the main road from the National Stud and the Curragh Racecourse, is a well-respected 220-acre part of the industry. The Aga Khan and his fellow shareholders hoped, and had every reason to believe, that Shergar would have an easy, fecund career there.

Security at Ballymany was loose. There was no reason to tighten it. Before the kidnaping, few stud farms bothered with surveillance cameras and elaborate fencing. Only two racehorses have been kidnaped, none in Ireland. Nelson Bunker Hunt's Carnauba was stolen by Italian terrorists in the summer of 1975, and a ransom of $300,000 was asked. Apparently, the ransom was not paid, and the horse was discovered in the winter in a holding pen in a butcher's shop in Milan. The mare Fanfreluche was stolen from her paddock at Claiborne Farm in Kentucky in June 1977 but was found several months later at a farm in Tomkinsville, about 150 miles away. The new owners said they had encountered Fanfreluche near their farm and took her in, renaming her "Brandy"; the horse, which was in foal to Secretariat, was returned and no charges were filed.

But never in Ireland, as Shergar's vet said. Not in a land where horses are an integral part of the cultural and economic life, lugging carts of peat along the bog roads and lugging silk-clad, whip-wielding jockeys along the tracks in Phoenix Park, Galway and Sligo.

Never in Ireland.

On Feb. 8, 1983, at around 8:30 p.m., a team of at least six masked gunmen in three vehicles drove up the long path to the main stud building at Ballymany. The gunmen burst into the home of the chief groom, James Fitzgerald. They locked his wife and seven children in a ground-floor room and demanded Fitzgerald show them to Shergar's paddock. One of the gunmen, using a two-way radio, called for a double-horse trailer and demanded Fitzgerald help guide the 5-year-old stallion into the vehicle. The trailer drove off with some of the kidnapers at 9 p.m. and the rest of the team left the farm with Fitzgerald an hour later.

The kidnapers told Fitzgerald they wanted a ransom of $2.6 million. They said they would kill Fitzgerald and his family if he dared call the Irish police, the Garda. With those instructions keenly understood, the kidnapers dumped Fitzgerald on the road and sped off. He was frightened, lost. Finally, after realizing he was in Kilcock, about 20 miles north of Ballymany, Fitzgerald phoned his brother Des, who drove him him back to the farm. Fitzgerald then phoned Ghislain Drion, the French stud manager, who lived nearby. Fitzgerald warned Drion not to call the Garda and, at 1 a.m., Drion tried frantically to reach the Aga Khan by telephone.

At about 3 a.m., Drion finally tracked him down in Switzerland. After a lengthy discussion, the Aga Khan told Drion that, despite the kidnapers' promises of violence, the police must be informed. But at that hour of the night and in a frazzled state of mind, Drion could only succeed in reaching a minister of finance who lived in Kildare, and then only at 4 a.m. By the time the matter was finally passed on to the Garda, the sun was up and the kidnapers could have been anywhere in Ireland or even abroad.

Fitzgerald, who had been born at Ballymany 50 years before, whose working life was devoted to Shergar, was terrified and confused. It took the police days to complete a comprehensive, lucid interview with him. FOUR DAYS IN WINTER

On Feb. 9 at around 4 p.m., the kidnapers contacted Drion at Ballymany Stud. Drion knew it was the kidnapers because they repeated a code word given to Fitzgerald the night before. Drion, who speaks with a thick French accent, pretended not to understand the caller so that the police could tap into the conversation. The kidnapers cut short the first call, then called back an hour later and repeated the original ransom demand.

In subsequent calls, they asked that they be able to call a number in France rather than Ireland. The police guessed that the kidnapers felt they would be safer negotiating that way. The syndicate stationed a representative at the Paris number.

Although there were various rumors of Shergar's whereabouts, no one believed the horse could have been spirited out of the country. Getting guns or drugs past customs officers is one thing; hiding a racehorse, an internationally famous one at that, quite another.

On Feb. 11, the kidnapers said they would leave proof at the Rossnaree Hotel on the Dublin-to-Belfast road that Shergar was still alive. Again, the kidnapers demanded $2.6 million.

On Saturday, Feb. 12, one of the five shareholders representing the syndicate collected proof of Shergar's status at the Rossnaree Hotel. The package contained several Polaroid photographs of the horse, some of which included a Feb. 11 copy of the Belfast paper, the Irish News. Shergar was still alive.

That day, the kidnapers called in the morning, saying they would kill the horse if the ransom was not prepared. They called back at 10:40 p.m., after the photos had been collected. Demands were repeated. The syndicate representative said the shareholders were not satisfied.

"Well, if you are not satisfied, that's it," the caller said.

The syndicate representative heard a click.

The line went dead.

The kidnapers had put an end to the negotiations and, probably, to a champion's life. THE SEARCH

Even though calls from the kidnapers had ceased, the police investigation continued in earnest. At the head of the operation was Chief Supt. Jimmy Murphy.

Wearing a rakish trilby hat and speaking with the sort of Kildare accent and quirky syntax often associated with the stock "stage Irishman," Murphy intrigued the press when he gave briefings on the station house steps. He was a "one-man media circus," according to one reporter.

"Murphy was optimism personified," said Michael O'Mahoney, an attorney for the syndicate. "It was the biggest story around and he was at the center. He was like a character out of Mickey Spillane and the press ate it up. Even as the weeks went by with nothing, he kept looking for another angle."

Although he seemed to revel in the initial attention, Murphy sometimes grew furious with the constant pressure in the press to find Shergar.

Capt. Sean Berry, who is secretary of the Irish Thoroughbred Breeding Association, recalled how Murphy played on the eagerness of his chroniclers.

"One winter morning, at about 5, Murphy summoned them all to the station," Berry said. "They rushed like madmen, some of 'em still in their pajamas, probably. And Murphy just came out on the steps and told them, 'Gentlemen, I just called you to say I have nothing to say.' "

By now, Murphy has grown weary of the case.

"I've ceased talking about it," he said in an exhausted tone. "The press made a field day of the case and of me. They always wrote about the horse and not the people whose lives were upset. There was a terrible fear in their houses because of the weapons the kidnapers used."

All leads and pursuits came to nothing.

In the first months after the kidnaping, the police asked tens of thousands of farmers to search for the horse in their stables, barns and fields . . .

An anonymous caller told the British Broadcasting Corp. that Shergar was somewhere in the Middle East . . .

A stud-farm employe contended Fitzgerald wasn't forced to help the kidnapers, but was part of the operation . . .

The police had photographs of three men, known as The Nose, The Jockey and The Guard, who may have been part of the conspiracy . . .

Three clairvoyants called. One said the horse was in Galway, one said Kilkenny, one said Mayo . . .

One theory had the horse with Muammar Qaddafi in Libya, another with hippies in southeast Ireland, another with gun runners in Marseilles . . .

All of it came to nothing.

"It all eased off after about three months," Murphy said. "Now I work on many, many things. You have to take into account more serious things. A person's life is involved in a murder, not a horse's. But we keep looking. You can never feel disspirited, though; you have to take into account the lapse of time. We follow up on anything. You can't call the case closed until it is." THE THEORIES

But for all practical purposes, the case is closed. Lloyd's of London paid $10.6 million to the owners of Shergar fortunate to be insured for theft. The "Shergar bureaus" closed. Stud farms in Kildare increased security measures. Fitzgerald and Drion quit talking to the press.

All that remains are theories and the ghostly possibility that Shergar is still alive.

The Wayne Murty Theory: The Aga Khan had an enemy in the bloodstock world named Wayne Murty, a mercurial Kentucky breeder who was fond of saying that he "began with nothing more than a ham sandwich."

In June 1978, Murty thought he had won a bidding war for 56 broodmares owned by a financially desperate French breeder and textile magnate, Marcel Boussac.

Thirteen days later, a French court ruled the Aga Khan had won the bidding contest. Murty, the court ruled, had bid $840,000. The Aga Khan's bid was for $1.4 million.

Murty was enraged. He went to the office of a junior French agricultural minister and, Godfather style, threw the severed leg of a horse on his desk. Murty demanded he be able to leave the country with "his" horses, but the bankruptcy court ruled otherwise. The Aga Khan, who owns at least 500 horses in England and France, had won. There were rumors that his gift to the French National Stud of three stallions worth $90,000 might have influenced the court.

Murty's defeat and resentment made him a "natural" suspect. A cause celebre in 1978, his picture appeared in European papers once more after the kidnaping.

But that theory turned out to be nothing but media speculation. There never was any evidence that Murty played any role in the kidnaping. The police, and eventually the press, dismissed the theory and contend that Murty had nothing to do with the kidnaping.

The IRA Theory: This is the official theory, the one held by the police and the Shergar syndicate. It is also the one most people in Kildare believe.

In a February report to shareholders, representatives echoed police reports that "the stallion was (probably) kidnaped by the IRA and died shortly afterwards."

Sources close to Ballymany and the police contend funding for the IRA from America had decreased and the group needed funds. They say some U.S. funding sources have dried up and a kidnaping would provide quick cash. They believe that Shergar injured himself and had to be "put down," or that the kidnapers had decided the situation was too perilous and killed the horse in frustration.

The report also contends the individual who led the kidnaping plot had been high up in the IRA leadership but since has been demoted. The police also tied the Shergar incident with the kidnaping of Don Tidey, a wealthy supermarket executive in November 1983. Tidey was rescued from an IRA bivouac a month later in Leitrim, a county with many Republican sympathizers.

The report lacks firm evidence against the IRA. Police say they received much information from IRA informants but, obviously, are unwilling to reveal evidence or sources.

A New York attorney who frequently defends IRA members in the United States was in Ireland at the time of the kidnaping and denies any Republican involvement.

"No one said 'boo' to me about it when I was there," the lawyer said. "I would have heard. That was no IRA operation. I'm sure of it. Why didn't they claim responsibility? They always claim responsibility for an action. The people who say that no Irish group would ever claim responsibility for a horse's death are wrong, too. When the Queen's horses were killed in front of Buckingham Palace, the IRA claimed credit. The idea that the IRA needed quick money is debatable, too. It's just not the case." AT GALWAY RACES

Shergar is a horse gone, a myth created.

For a thousand reasons, the loss of a poet such as Yeats is acute. One is that he who took delight in that which "makes all of us of the one mind/The riders upon the galloping horses" could not write the lines and cadences of Shergar's legend.

We are left with wonderers and, alas, journalists.

Capt. Luke Mullins, who led the preparations for this month's Galway Races, said, "So many people whose job it was to know what became of that great horse. How can I guess? What chance have I?"

"It's one of those stories that will never die," said Peter Murtagh, who spent months tracking Shergar for the Irish Times. "Every year, we'll get another one. Shergar's in Saudi Arabia. Shergar's racing somewhere in Kentucky. He's with Qadaffi in Libya. Until they find a body in a bog or until someone opens a tin of horsemeat with Shergar's name written on it, people will always wonder."