BOSTON (AP) _ Five men including two Irish citizens were sentenced to prison Friday for their part in a failed scheme to smuggle a surface-to-air missile and other arms to the outlawed Irish Republican Army.

Four of the five defendants, U.S. District Judge A. David Mazzone said, played ''late and minor roles in the conspiracy, but the possible consequences of the plan was just clearly too dangerous to ignore.''

The defendants, who pleaded guilty earlier to conspiracy to illegally export arms, were accused of trying to smuggle 100 M-16 rifles, 100 rounds of ammunition and a Redeye surface-to-air missile to the IRA.

They and three others were arrested in May at an airport west of Boston as they were about to load the weapons aboard a jet that used by the Justice Department for the sting operation, prosecutors said.

The judge sentenced John A. MacDonald, 37, to the longest sentence, 18 months. He was described by the prosecution as a ''go-between'' who introduced an undercover FBI agent to the IRA connections.

Mazzone sentenced MacDonald's brother, Stephen, 35, and Roy Paul Willey, 28, to one-year sentences, half of which was suspended, and two years of probation with drug and alcohol counseling. James Boyle, 36, and John Fitzgerald, 30, both Irish citizens living in the Boston area, were sentenced to six months in prison.

Sentencing for a sixth defendant, Michael P. McLaughlin, 27, brother of a Boston City Council member, was postponed for a week after his attorney said he was undergoing a psychiatric evaluation.

Defense attorneys described the men as minor players in the conspiracy, saying they were led into it because of drinking problems.

''These men are the Budweiser brigade of the Irish Republican Army,'' said attorney William Brown, who represented Stephen MacDonald.

Referring to a co-conspirator who was convicted last month, Brown said: ''All Noel Murphy had to do was go across the street to the Shamrock bar and offer to pick up the next round of beers to get these people to do what he wanted them to do.''

The men were accused of acting as a loading crew for Murphy, 26, of County Kerry, Ireland, and alleged IRA contact Cairan Hughes, 24, of Belfast, Northern Ireland, who were convicted last month of conspiracy to violate federal arms export and firearms laws and unlawful dealing in firearms. They are scheduled to be sentenced Nov. 26.