Irish Republican Army guerrilla Thomas McMahon, 31, was sentenced to life imprisonment today for the Aug. 27 assassination of Lord Louis Mountbatten, a cousin of Queen Elizabeth II and Britain's last viceroy of India.

Another defendant, 24-year-old Francis McGirl, was found innocent of the slaying, but the special antiguerrilla court -- composed of three judges and no jury -- ordered him to stand trial Jan. 21 on charges of belonging to the outlawed IRA's Provisional wing.

Presiding Judge Liam Hamilton refused to allow McMahon, a longtime Provisional IRA explosives specialist, to appeal the life sentence.

Both men denied they killed the 79-year-old soldier-statesman, who died when a 50-lb. bomb exploded aborad his boat in Donegal Bay during a vacation in the Irish Republic.

Also killed were Mountbatten's 15-year-old grandson, Nicholas Kratchbull; Lady Brabourne, mother of Mountbatten's son-in-law; and a 15-year-old boatboy.

The assassination of Mountbatten, the most prominent Briton slain since the IRA began its battle to end British rule in Northern Ireland, led to a major tightening of security in Ireland, as well as increased cooperation between the British and Irish governments to curb the guerrillas.

Irish police will keep open the Mountbatten file in the belief that up to seven other men were involved in the murder plot.

"We're confident more arrests will be made in this case soon," one police source reported.

McMahon will serve his sentence in the Irish Republic's top security prison west of Dublin, where more than 150 convicted IRA men are held. CAPTION: Picture, FRANCIS McGIRL . . . not guilty of Mountbatten slaying