A federal jury in Brooklyn yesterday convicted four men of conspiring to smuggle arms, including Red Eye surface-to-air missiles, to the outlawed Irish Republican Army.

The verdicts came after five days of deliberation and nine weeks of testimony in which the jurors heard defense arguments about alleged CIA involvement, FBI entrapment, diplomatic pressure from the British government and contentions that two of the defendants had been tortured in a Northern Ireland jail.

The trial of the four--an Irish American and three Irish aliens--had been watched closely in London and Washington because five men recently were acquitted in the same courthouse in a similar gun-running case after basing most of their defense on charges of CIA involvement.

British intelligence sources estimate that 90 percent of the terrorist army's weapons--ranging from French explosives to Soviet AK47 assault rifles as well as U.S.-made arms--are smuggled into Northern Ireland by IRA supporters in the United States.

In a spate of federal trials over the past 10 years, about 30 Irish-Americans have been charged with gun-running and, before yesterday's verdict, about 20 had been convicted.

But the trials have become increasingly emotional, with crowds of relatives and IRA supporters packing the courtrooms and chanting "Up the IRA!" as the verdicts are read. Authorities in Britain and the United States had begun questioning whether New York juries could continue to bring in convictions, even when the gun-runners were caught red-handed.

As yesterday's verdict was read, sobs were heard among a crowd of about 100 spectators, most of whom had sat through all of the 2 1/2-month trial. The wife of one of the defendants broke into tears, slammed her fist into a wall and cried, "Margaret Thatcher wanted this"--a reference to the British prime minister.

The gun-running trials, aimed at cutting off the IRA's primary source of weapons, also have caused a wide rift among Irish Americans, particularly in New York.

Earlier this year the controversy touched such hallowed Irish-American institutions as the Roman Catholic Church and New York's St. Patrick's Day parade when Michael Flannery, founder of the Irish Northern Aid Committee (Noraid), was named the parade's grand marshal.

Flannery, 81, an outspoken IRA backer, was among the five men acquitted in the earlier gun-running trial when the jury apparently accepted defense contentions that the CIA was involved with the alleged weapons smugglers. The CIA denied involvement.

But Flannery's appointment to lead the festive Irish parade focused sharp attention not only on his activities but also on those of Noraid. For the first time several leading New York politicians, including Sen. Daniel Patrick Moynihan (D-N.Y.), refused to march in the parade, and New York's Cardinal Terence Cooke declined to give the grand marshal his traditional blessing.

Noraid, which is registered as a foreign agent, has raised more than $1.5 million in the United States. The British suspect that some of the money is funneled into the arms trafficking, a contention that Flannery strongly denies. The stated purpose of the Noraid fund-raising is to assist families of IRA prisoners in Northern Ireland.

Flannery staunchly denies that either he or Noraid has been involved in gun-running. But he makes no bones about his support for the trafficking. He says that "getting guns over to the Irish Republican Army is a very good thing," and that George Washington "had to do the same thing when he was in revolt here against the same enemy."

Among those convicted yesterday was Andrew Duggan, 49, of New York, who served as an aide to Flannery as grand marshal. The prosecution's chief witness, FBI informer Michael Hanratty, testified that he had 80 meetings with Duggan in the year before his arrest and that arms purchases were discussed at most of the meetings.

The aliens convicted with Duggan were Gabriel Magahey, 39, Colum Meehan, 34, and Eamon Meehan, 30, all from Belfast and living in New York.

The four men face sentences of up to 10 years for conspiring to smuggle $1 million worth of arms, including surface-to-air missiles to be used against British helicopters. The FBI said few of the weapons, and none of the missiles, reached the IRA.

The jury heard highly emotional testimony from the Meehan brothers, who contended they had been tortured during 2 1/2 years of imprisonment without trial in Northern Ireland's Long Kesh barracks.

The prosecution played hours of wiretap and videotape evidence, including videotapes of the defendants loading weapons into a van and transporting them to a house in Brooklyn.

Even so, the jury spent almost five days in deliberation, repeatedly returning to the courtroom to hear and view the tapes, before delivering the guilty verdicts.