Earlier this month, a foreign visitor to any of Ireland's scores of stud farms would have been surprised to find the gates open, horses on display and an open invitation to come in and look around.

"We've always been proud to show off our horses. We breed the best in the world and don't believe in hiding them away," the president of the Irish Thoroughbred Breeders Association, Capt. Sean Barry, said tonight. "It's always been a bit like going to the zoo."

Since last Tuesday's kidnapping of the $13 million stallion Shergar from his stall at the Ballymany Stud Farm, however, paddock gates around the country have swung shut, and armed guards are now patroling the vast horse farms that dot the pastoral countryside of County Kildare.

In the past seven days Barry reportedly has raised $100,000 in reward money for the safe return of the stallion. When Barry gets home at night, he finds his mailbox stuffed with letters containing religious medals and saints' relics--all sent by people who say they are praying for the horse's safety.

Meanwhile, the countrywide search for the 5-year-old Shergar now involves every police station in Ireland plus Northern Ireland's rural Ulster constabulary. After several quiet days with no new developments, police announced today they were looking for three men believed to be involved in the abduction.

They have code-named the trio The Jockey, The Nose and The Guard. The Jockey is a small man; The Nose has a large nose; The Guard was spotted near the 220-acre Ballymany Farm sporting a dark uniform and peaked cap. Police believe he may have been impersonating an officer during the kidnapping.

The week-long search has been marked by conflicting reportsfrom police and stud-farm employes and by a series of apparently hoax phone calls, first demanding ransom and then alleging the horse was dead. Finally, rumors have surfaced that secret negotiations are taking place in London between the kidnappers and an intermediary for Shergar's owner, the Aga Khan.

The drama, which has the Irish public riveted to television and radio for up-to-the-minute reports, began a week ago at 8:30 p.m. when Shergar was spirited out of his stall by a gang of five to eight armed men.

The masked men reportedly burst into the nearby home of chief groom John Fitzgerald, locked his wife and seven children into one room and took Fitzgerald to the stud farm at gunpoint.

Fitzgerald told police the gang demanded he lead them to Shergar. They loaded the horse into a trailer, forced Fitzgerald onto the floor of a car and dumped him about eight miles from the farm on a road to Dublin.

Fitzgerald told police he could not remember the number of men in the gang, what they looked like, how they sounded or the color of the car. There was a four-hour gap between Fitzgerald's alleged abduction and the time the police were notified.

A stud-farm employe today disputed official police reports, charging that the gang made its way to Shergar's stall without the guidance of the chief groom and that Fitzgerald rode in the car like "a normal passenger." Fitzgerald has not been arrested but has been questioned by police several times.

In the days following the kidnapping a series of phone calls to the BBC and national Irish newspapers indicated the kidnappers were asking for ransoms of $70,000 to $4 million. On Friday morning the call the Irish public was dreading, came. A voice said the horse had injured himself and had been "put down."

But when police failed to find Shergar's body in the location specified by the caller, they assumed all of the calls up to that time had been hoaxes, since they had come from the same man using a secret code word.

With solid clues scarce, the police followed tips from hundreds of callers, including three clairvoyants who claimed the horse was being held in Galway, Mayo and Kilkenny.

The search seemed futile, however, until a description of the suspected trio was released today.

The object of the massive search is a beautiful bay stallion with a white blaze on his nose and white stockings. Shergar, who won five major races as a 3 year old, was retired to stud after winning the Epsom and Irish derbys in 1981. The Aga Khan syndicated Shergar last year to 34 investors who paid about $500,000 each for a share in the stallion. The Aga Khan reserved six shares for himself.

Just how successful Shergar has been as a sire remains to be seen. Four days before his kidnapping the first of his offspring was born, a bay colt, who is the spitting image of his father. One thing is known about the Irish stallion--he was unusually fertile, putting 42 of his first 44 mares into foal. Shergar's fecundity has prompted Irish breeders to say he may be the greatest stallion in history. "Greater than Secretariat," said one.

With the horse-breeding season just beginning and extending until June, Shergar was expected to be quite busy over the next five months. Four mares arrived in Ireland last week to be bred with the kidnapped champion; others, including several from the United States, were due next week.

The kidnapping of Shergar has Irish horse breeders looking nervously at the future of the burgeoning bloodstock industry. Revenue from Irish horsebreeding is estimated to have reached $2 million last year.

Breeders fear the kidnapping--combined with Ireland's reputation as a country plagued by terrorists--may frighten prospective clients and make them decided against sending their mares to Ireland to be bred.

"It's too soon to panic," warned Lord Killanin, former president of the Olympic committee and a member of the Irish government's Commission on Bloodstock in a telephone interview tonight. "With every hour that passes I become more concerned about his safety, but I still trust he will be returned safely."

Killanin speculated that the abduction might not be a simple extortion bid, but the result of a feud within the horse-breeding community.

"We simply don't know," he said, "but it does seem the kidnappers were quite familar with the stud farm and with horses in general."