It was to be the most audacious operation in the history of the Irish Republican Army -- the killing of Prince Charles and his wife, the Princess of Wales -- and the organization needed someone it trusted.
It turned to Sean O'Callaghan, a member of the group since his teens who had proved his loyalty by building bombs, attacking police precinct houses with mortars, conducting robberies and killing two members of the security forces.
He was sent to London and provided with delayed timing detonators and 25 pounds of a powerful mining explosive called Frangex gelignite. The hit was scheduled for a night when the newly married Prince and Princess were to attend a charity benefit at a West End theater by the rock group Duran Duran.
Mr. O'Callaghan cased the theater, found the spot in the wall of a bathroom stall near the Royal Box where the bomb would fit and learned the time when cleaning women opened the restroom, giving him the two hours he would need to plant the device. ''It would have worked, it had a high chance of success,'' Mr. O'Callaghan said in a recent interview.
The 1983 plot failed, however, and one reason was that Mr. O'Callaghan was an informer for the Irish police special branch and British intelligence, the highest ranking I.R.A. man, it later developed, ever to betray the clandestine organization from within.
Despite his cooperation with the authorities, Mr. O'Callaghan was sentenced to multiple life sentences for two homicides and 40 other admitted acts of terrorism. Now, after serving just eight years, he has been freed, just in time to enter the debate over Northern Ireland as an implacable and vitriolic foe of the I.R.A.
In particular, he has attacked the effort by Gerry Adams to gain respectability and a role in the peace process for Sinn Fein, the I.R.A.'s political wing, which Mr. Adams leads.
''I wish I could be confident about the prospects for peace, but I have sat behind too many closed doors with too many of the present leadership to be conned by the smooth presentations,'' said Mr. O'Callaghan, a former member of the Sinn Fein executive council.
Mr. O'Callaghan was released from prison on Dec. 6 under a rarely used procedure requiring approval of the Queen. The timing was critical for the British Government, for it introduced into the debate an insider who is forcefully making the same argument put forward by John Major, the British Prime Minister.
Mr. O'Callaghan may be banking on the notion that maintaining a high profile is his best protection, that killing a public figure would not be in the best interests of a group seeking admission to peace talks.
Mr. Major believes the 18-month I.R.A. cease-fire that ended with the bombing of a London office building in February was a ruse through which the organization gained political standing while it retooled its terrorist apparatus. He has rejected conditions set by Mr. Adams and John Hume, the mainstream Roman Catholic leader in Northern Ireland for Sein Fein's participation in peace talks, demanding that the I.R.A. provide evidence of a ''lasting cease-fire.'' Mr. Adams and Mr. Hume argue that Mr. Major is sabotaging their effort to rebuild the peace process and that a ''precious moment'' is passing.
Enter Mr. O'Callaghan, who over the past two weeks has voiced shame at what he did for the I.R.A. and urged Britons not to believe the pacific claims of Sinn Fein. He has testified before the Commons, given radio interviews and written articles for The Sunday Times, the newspaper that first made his double life public in 1992.
Mr. Hume, who has staked his public career on establishing all-party talks on the future of Northern Ireland, has reacted to Mr. O'Callaghan with fury.
Mr. O'Callaghan said he had accepted no arrangements and no money for his present actions. He said that the British Government offered him its normal witness protection program -- a new identity, a $75,000 house, nine months' police protection and a job -- but he turned it down.
Mr. O'Callaghan's account of his I.R.A. career could not be independently corroborated. The organization has not challenged his statements, or the admissions that led to his prison term.
A sad-eyed man who looks older than his 42 years, Mr. O'Callaghan was born into a home in Tralee, County Kerry, run by a strong-willed father who was an I.R.A. member.
His home was full of books, and he read Irish history and Irish politics, war books, adventure thrillers and the writings of Ernesto (Che) Guevara. He said he became impassioned for the cause in the late 1960's after seeing television footage of the troubles in Northern Ireland and meeting Catholic refugees burned out of their homes in Belfast.
When representatives of the militant Provos, the Provisional I.R.A., came around looking for recruits in 1969, Mr. O'Callaghan was ready. He was taken to a nearby farm where he watched a group of people shoveling away grain to reveal 30 to 40 guns. He helped clean and package them for shipment to the North.
A country boy of 15, he became an explosives and firearms instructor at weekend camps in the hills around Tralee. One Sunday afternoon in a shed behind his house he dropped some sulfuric acid into an explosive slurry he was mixing, and the blast blew him out the back door.
He was jailed for six months for possession of explosives. On his release he was put in charge of a bomb factory in the South where he was responsible for turning out ammonium nitrate distilled from boiling fertilizer in oil drums.
In May 1974 he was sent to the ''hard edge'' in Northern Ireland and on his first night took charge of the mortars in a Provo assault on an Ulster Defense Regiment barracks. An officer, Eva Martin, 28, was slain in the atttack. At age 19, Mr. O'Callaghan was a killer.
Through the summer he took part in robberies and bombings, and in August he was ordered to commit a face-to-face killing. Peter Flanagan was a detective inspector with the Royal Ulster Constabulary special branch, and the I.R.A. had established that he went each day to a pub in Omagh, a town in County Tyrone, Northern Ireland. He was reading the racing pages of The Irish Independent when Mr. O'Callaghan drew his short-barreled Magnum. He shot him eight times.
Mr. O'Callaghan marks that incident as the moment he decided to leave the organization. He went to England, set up a cleaning business in North London and married a Scottish Protestant woman. But he kept thinking about Ireland and was influenced by criticism of the I.R.A. by the Irish writer Conor Cruise O'Brien. He returned to Tralee in 1979, formally rejoined the I.R.A. and simultaneously told a man he trusted from the Irish police force that he wanted to inform on his comrades.
Over the next three years he delivered information to the Irish police as he rose into the command structure of the I.R.A. in Ireland. In 1982 he was assigned to the ''England Department,'' which was run from Dublin, and put in charge of the proposed assasination of the Prince and Princess of Wales. Just three years earlier, the I.R.A. had killed Earl Mountbatten, Prince Philip's uncle.
After discovering that the Prince and Princess of Wales would attend a rock concert on July 20, 1983, Mr. O'Callaghan arranged for shipments of explosives and detonators and went to London to supervise the planning. As the date got closer, he told his Irish police handlers and M.I.5 officials that he needed an artful way to sabotage the effort without compromising his cover.
Under the plan, Scotland Yard leaked a story to the London newspapers that it was hunting for Mr. O'Callaghan in connection with a plot to assassinate a member of Margaret Thatcher's Government. Mr. O'Callaghan had already slipped out of England for France when the stories appeared. It was too late for a substitute bomber to be put in place. When Mr. O'Callaghan returned to Ireland, the I.R.A. chiefs congratulated him for eluding the police. ''The amazing thing is that my street 'cred' actually went up,'' he said.
Mr. O'Callaghan became a local councilman for Sinn Fein and in 1985 was named to its national executive council. He had kept feeding information to the Irish police during this period, leading to the arrest of some Provos and in one case the seizing of seven tons of arms off Ireland's southwest coast.
By November 1985 he decided that he could not safely pull off the deception much longer and so he flew from Cork to London. There he was debriefed by an M.I.5 agent, who told him he could not obtain a pardon for him despite his work for the British Government. In 1988 he walked into a police station in southern England and admitted his crimes.
His undercover work went unmentioned in his trial at Belfast Crown Court. He pleaded guilty to 42 crimes and was sentenced to a total of 539 years. By 1992 he realized he could be of no more use as an intelligence source and he contacted The Sunday Times to get his story out and take his anti-I.R.A. campaign public.