In its continuing war with the British government, the Irish Republican Army showed this week that it could still mount a spectacular feat of defiance, making a mockery of security at an "escape-proof" penal institution and attracting the attention of the world.

But Sunday's mass breakout by 38 men from the Maze Prison near Belfast may have been as desperate as it was daring. British authorities believe that the IRA and its Protestant paramilitary adversaries are reeling from the effects of the police's dramatic recent successes with the use of informers--"supergrasses," they are called--whose uncorroborated testimony could send them to prison for lengthy terms.

In the 15 bloody years of this latest revival in Ulster of age-old national and religious conflicts, security forces have searched for effective ways to reduce violence, thereby fostering an atmosphere that would ease the continuing political deadlock. There are again faint signs of progress on the political front--Prime Minister Margaret Thatcher will be meeting with Irish Prime Minister Garret FitzGerald later this fall and renewed efforts are being made to establish an all- Ireland forum including Northern Ireland's Protestants.

But it is the battle against terrorism that is always the more immediate issue in Northern Ireland and where some gains--until Sunday, at least--were clearly being made.

At the center of the drive is a series of about two dozen "supergrass" trials that began last spring, each featuring scores of defendants named by their former comrades. These informers--about 30 so far--are officially designated as "converted terrorists" and are kept in protective custody except for their tense courtroom appearances as the state's star witnesses.

In all, according to a Royal Ulster Constabulary spokesman, more than 300 persons have now been charged with terrorist offenses since the wide-scale use of informers began about 18 months ago. There is an ignoble history of informers in the Irish Republican movement, but what made this batch of defectors unique was that they were prepared to implicate large numbers of their colleagues. So far, Protestant paramilitary informers have named only a quarter as many defendants as IRA groups have.

In one IRA trial this summer, 35 people were convicted and 22 of them received sentences totaling over 4,000 years--almost entirely because of the testimony of Christopher Black, a long-time IRA guerrilla who was granted outright immunity against prosecution.

Those facing trial, the police spokesman said, now represent a "significant proportion" of the active strength of both the IRA and Protestant paramilitary groups. And while, especially in the aftermath of the Maze escape, no one would hazard that the sectarian struggle is ending (or that IRA support is declining), police figures on violence in every category have dropped this year to their lowest point since 1970, a total of 47 deaths or about half the number of last year. Overall incidents of trouble are also down.

There may be other factors responsible for the improvement--public weariness of the pain and hardship caused by the conflict is certainly thought to be one. But the view of legal specialists, clergy and moderate politicians, even those who find aspects of the informers program objectionable, is that it has had a major impact.

Arrests have depleted the paramilitary forces in Belfast and Londonderry in particular, police sources report. But throughout the province, they say, the organizational and planning structure of the groups has been damaged by the discovery that so many people are prepared to abandon one of the main pillars of a guerrilla group--total loyalty.

The 30 "supergrasses" recruited thus far have received promises that, in addition to immunity in most cases, they and their families will be resettled in a "friendly country," provided with a new identity and a job. Police deny allegations made by the IRA and some prisoners who have been unsuccessfully approached that bribes in "five or six figures" are being offered.

Secretary of State for Northern Ireland James Prior has acknowledged that hundreds of thousands of pounds will be spent on the "supergrass" program to meet the expenses of the informers. "There is money involved when you move them abroad and set them up in a house," he said, "but there are no large sums paid in bribes." The money was well spent, he insisted, to destroy the IRA's "monolithic structure."

Police sources and a prominent Belfast lawyer maintained in interviews that many of the informers are motivated by a genuine conversion against violence after years either in prison or on the run. "They hit their thirties or forties and face spending the rest of their lives in custody," the lawyer said. "Suddenly it doesn't make sense anymore."

Nonetheless, the use of inducements to "supergrasses" undoubtedly has its unsavory side and many of the legal shortcuts involved are proving controversial.

Even admitted murderers have been given extraordinary leniency in return for talking. An informer from the Irish National Liberation Army, convicted of five murders, six attempted murders and 74 other offenses, had his life sentence reduced to an undisclosed minimum--thought to be 10 years--to be spent in an English jail.

However, it is the judicial procedure in the trials that has aroused the most criticism.

For a decade, terrorist cases in Northern Ireland have been heard without juries on the grounds that the chances for political interference and intimidation are therefore greatly reduced.

The judges in "supergrass" cases have determined, moreover, that there is precedent in British common law for permitting uncorroborated testimony against former accomplices. This means that the only evidence in many cases is the word of a single informer.

"Settling old scores" is not the purpose of justice, one Protestant clergyman observed this week, and the chances of wrongful convictions are far greater than in any British trial.

For instance, in the Christopher Black case, according to Mary Holland writing in the current New Statesman magazine, one of the accused "was able to produce more than 30 people in court to substantiate his alibi" but was still convicted.

Finally, there is the real possibility that dangerous criminals will manipulate the police and go free. Under the immunity laws, a "supergrass" who recants his testimony cannot be reindicted. This has already happened in six cases, according to the Belfast lawyer--or only three times, according to police.

Pressure on the defectors to withdraw their testimony is intense. Threats are made against their families and some have been disavowed by their wives and children. Patrick Gilmour, the 62-year-old father of a "supergrass," was released by the IRA Monday night after being held captive for 10 months in a futile effort to force his son to recant.

Unsatisfied with the outcome of the elder Gilmour's ordeal, neighbors and relatives of men the son has implicated besieged the family apartment overnight, shattering windows and breaking down the door. At dawn the family fled to a hiding place. In a television interview, Gilmour denounced his son as "a coward."

Aside from the enormous emotional strains which the "supergrass" phenomenon imposes, there is a feeling that the IRA and other groups may be driven to ever more gruesome acts by the police success to show that they have not been defeated. Gerry Adams, a top official of Sinn Fein, the IRA's political wing, told interviewers last week that the organization "is not now, and has never been, on the run."

By that reckoning, the well organized and well executed Maze breakout, with at least nine murderers among the 19 fugitives still at large, could mean a resurgence of IRA activity and further trouble ahead for unhappy Northern Ireland.