DUBLIN, April 4 - Denis Donaldson, a former member of the Irish Republican Army who was exposed last year as a British spy, was found shot dead Tuesday evening at his isolated home in Donegal.
Ireland's justice minister, Michael McDowell, confirmed that Mr. Donaldson had taken a shotgun blast to the head, and that his right forearm was ''almost severed,'' a mutilation similar to those inflicted on I.R.A. informers throughout Northern Ireland's sectarian conflict.
Mr. McDowell said Mr. Donaldson, 55, had been tortured in his home near Glenties, in northwest Ireland, The Associated Press reported.
In a statement, the I.R.A. denied any involvement, but the killing threatens the fragile equilibrium that has lasted since last summer, when the group declared an end to its war against British rule in Northern Ireland.
In 2002, Britain accused Mr. Donaldson of spying for the I.R.A. by stealing documents from government offices at the Northern Irish parliament.
At the time, Mr. Donaldson was receiving paychecks from the British intelligence agencies MI5 and Special Branch while he was serving as Sinn Fein's chief administrator for the power-sharing provincial parliament.
The charges brought down the coalition that was formed under the Northern Ireland peace accord of 1998.
His killing came just two days before the Irish and British prime ministers, Bertie Ahern and Tony Blair, are to meet in Northern Ireland to discuss restoring the local government.
Mr. Donaldson's spying for the British became public last December after the 2002 spying charges against him and two others were dropped suddenly by the British government.
After he was informed that the charges were being dropped, he admitted his dual role to his Sinn Fein colleagues, an announcement that came as one of the movement's most embarrassing scandals in years.
When he confessed in December, he told the state broadcaster RTE, ''I was recruited in the 1980's after compromising myself during a vulnerable time in my life.''
He added, ''I apologize to anyone who has suffered as a result of my activities, as well as to my former comrades, and especially to my family, who have become victims in all of this.''
Gerry Adams, president of the I.R.A.'s political wing, Sinn Fein, after personally offering his condolences to Mr. Donaldson's family, said: ''It has to be condemned. We are living in a different era, and in the future in which everyone could share.
''This killing seems to have been carried out by those who have not accepted that.''
Ian Paisley, the leader of Sinn Fein's Protestant rivals, the Democratic Unionists, said the killing ''has put a dark cloud'' over the talks Thursday between Mr. Ahern and Mr. Blair.
''Eyes will be turned towards I.R.A./Sinn Fein on this issue,'' Mr. Paisley said.
As the province's largest party, the Democratic Unionists are exercising a de facto political veto by refusing to share power with Sinn Fein.
Mr. Blair and Mr. Ahern plan to restart the fledgling legislature in May, despite Protestant worries about the continued existence of the I.R.A.
Many Northern Irish republicans, who want the province to break away from Britain and join Ireland, might be happy to see Mr. Donaldson dead, and Mr. Adams suggested that his death might have been the work of British intelligence agencies seeking to move blame back toward the republicans.
Mr. Donaldson had earlier earned his bona fides as Mr. Adams's cellmate when they were jailed in the 1970's. He was also arrested after training in guerrilla warfare with Hezbollah in Lebanon in the 1980's.
Since his admission of spying and his expulsion from Sinn Fein, Mr. Donaldson seemed to age decades. When he finally admitted that he had been spying for the British, he fled his home in Belfast to a remote part of County Donegal, and a cottage that had no electricity or running water.
Hugh Jordan, a journalist who found Mr. Donaldson and reported on his situation recently, told the Press Association in Britain that ''he looked like a hunted animal.''
''He was extremely depressed,'' Mr. Jordan said after the killing. ''The nerves in his eyes were trembling. He seemed like a man who didn't think he would come to any harm. He did not see his life to be in any danger, but felt the only future he had was where he was, living in that dreadfully squalid situation.
''He was alone and threatened no one. He was no harm to anybody.''
Photo: Denis Donaldson, in photo from December. He spied on the Irish Republican Army for the British. (Photo by Paul Faith/Press Association)