The Gardai, Ireland's national police force, has been rocked by charges that a senior officer has faked finerprint identifications.

The scandal could block any solution to the murder 18 months ago of Britain's ambassador to Dublin, casts a cloud over at least four other murder cases and raises the probablity that innocent men are languishing in Irish jails as victims of falsified evidence.

The affair has been a terrible embarrassment to the 9,000-man force, widely regarded as cleaner than most, and to this small country's self-esteem. In Ireland, as in many other countries, police testimony on fingerprints is usually accepted as gospel and goes unchalenged in courts.

The Gardai and its iron-willed commissioner, Edmund Carvey, successfully buried the affair for months. In the best bureaucratic tradition, Garvey demoted the two sergeants who initiated the accusation about the alleged faking. Their friends then went to The Irish Times where two young reporters, Don Buckley and Joe Joyce reported the story.

Now Ireland's Director of Public Prosecutions, the equivalent of the District Attorney in the U.S. has ordered an investigation to determine what criminal charges should be brought against the experts accused of faking. The official, Eamonn Barnes, has already demonstrated that he is tough and thoroughly independent. Nevertheless, he has no investigators of his own and depends on the police for his investigations.

Even so, knowledgeable sources here are convinced that between the Irish Times reporters and the demoted Gardai sergeants at least some of the Fingerprint Section's dirty linen is bound to be washed in public.

The principal figures in the case would not talk for quotation. This account is based on private conversations with most of those centrally involved.

The misidentifications came to light following the most politically sensitive murder in recent Irish history. On July 21, 1976, a land mine blew up and killed the British Ambassador, Christopher Ewart-Biggs and his secretary, Judith Cook, while they were riding in a car.

There was enormous pressure from London and from Prime Minister Liam Cosgrave in Dublin to solve the double murder for which the Provisional IRA claimed "credit" The police promptly picked as their prime suspect an Ulster IRA man who left for the United States the day after the killings. But there was no evidence.

A worker's hard hat was found at the bomb crater. But a fingerprint expert, Sgt. Michale Diggin, examined the helmet and found nothing on it.

Several weeks later, however, the boss of the Fingerprint Section, Insp. William Byrne, told the commissioner that he had found a print on the hard hat. Even better, it matched the prints of the Ulster suspect. Congratulations flowed from London to Dublin and the story was given to the press.

Diggin's professional pride was bruised. How could he have missed the print? He took another look at the helmet and discovered that it did contain a fingerprint after all. But it was his own. He checked his findings with the No. 2 man in the section, Sgt. Patrick Corless, also a fingerprint expert. Corless was in no doubt. The print was Diggin's own.

This was serious. Fingerprints experts are convinced their is an exact science. To establish an identification, the Gardai requires 12 points of similarity and none of dissimilarity in the unique ridges and loops on every person's fingers. In the view of Corless and Diggin, Byrne could not have made a mistake; his misidentification was deliberate, a charge Byren stremulously contests.

The pair confronted their superior. But Byrne dismissed them. He in turn got backing from another fingerprint sergeant.But Diggin and Corless persisted and still others in the section backed them.

To add to everyone's dismay, police in Ulster arrested the Gardai's prime suspect. But because of the battle over the prints, the Dublin police had to tell their Belfast colleagues to let the man go. The murky state of evidence may save Ewart-Biggs' killer from ever being caught.

Commissioner Garvey, frustated at losing his man, ordered his own internal inquiry into the affair. It has never been made public. But all four men - Byrne and his backer, Diggin and Corless - were demoted to routine tasks. Later, Insp. Byrne alone was restored to his job and he says he is the boss of the Fingerprint Section again today. His two accusers, however, continue at meaningless filing and copying tasks.

Corless and Diggin were determined to press on. Corless is known to have been aware of the consequences from the start but belived he had no choice but to root out what he regards as rot. The pair examined four other murders where Byrne had made a finger or palm print identification. All four, the two sergeants concluded, were mistaken.

Ranking Dublin police insist that Diggin and Corless have "scraped the bottom of the barrel," that there are no more dubious identifications. But the two sergeants have been cut off from the files and so nobody knows whether there are any more cases.

As for Byrne, he acknowledges only that he made a human mistake in the Ewart-Biggs case. He has made no public comment on the other four.

Frustrated within the department, Diggin and Corless brought their charges to Barnes, the director of prosecutions. They told him they will swear in court that the identifications were deliberate.

There is no suggestion that conventional corruption, the payment of bribes, is involved. Some police believe that over-eager officers, anxious to clear the books and curry favor with superiors. may have cut corners.

Prosecutor Barnes expects to get his inquiry report in four weeks. He will then decide whether criminal charges should be brought. And despite his independence Barnes has instructed the police investigators to examine only the five cases cited by Sgts. Diggin and Corless. He has no intention of looking at other cases where fingerprint evidence by Byrne led to a conviction.

In the United States, a wrongly convicted prisoner can file a motion seeking a new trial on the basis of new evidence. But neither British nor Irish law allows for this. Only the government can make such a move at its own discretion.

So unless Barnes or other government officials change their minds, the possibility remains that men will stay in jail on the bassis of falsified identifications.