Within the IRA and Sinn Fein, Denis Donaldson was regarded as such a staunch republican that his incorrigible womanising rang no alarm bells, even in organisations forever suspicious of infiltration by the security forces.
Donaldson's frequent approaches to women were so well-known in republican circles that they were not regarded as rendering him open to recruitment as an agent. Indeed, one of a number of mysteries that remain about the Donaldson affair is exactly how Special Branch recruited him 20 years ago. He gave no details, apart from saying he was recruited "after compromising myself during a vulnerable time in my life".
According to one who knew him well, Donaldson was both charismatic and irritating, causing some to like him but others to dislike him. "He had been around a few corners, enjoyed the friendship of the Sinn Fein leadership and was always believed to be a 'sound' republican," the source said. The Irish feminist Marie Mulholland wrote of him: "Denis stood out, all five foot nothing of him. Yes, he was a small man but somehow it never seemed to matter because he had charm - buckets of it. Not the smoozing of an operator, but real charm. It worked wonders with women and Denis loved women - lots of them."
Donaldson's career in the IRA and Sinn Fein spanned the entirety of the Troubles, from the rebirth of the IRA in 1970 to Sinn Fein's growth into Northern Ireland's largest nationalist party.
A member of an old republican family, he was identified with crucial events in recent republican history. In 1970 he helped give the IRA new credibility as the defender of vulnerable Catholic communities when he helped fend off a loyalist assault on the Short Strand district of Belfast.
Later, he was photographed inside the Maze prison with Bobby Sands, the most revered republican martyr, their beaming faces conveying IRA prisoners not as sinister terrorists but as friendly comrades.
Donaldson travelled the world for the IRA, visiting Europe and many parts of the Middle East. In Lebanon he was closely involved in attempts to secure the release of the Belfast-born hostage Brian Keenan.
He was also sent to the United States, serving as a contact point with the important Irish-American community. In his travels he is thought to be have acted in a dual role, seeking to win friends for Sinn Fein within groups such as the PLO, but also looking out opportunities for the IRA to procure arms abroad.
More recently he became an important cog in the Sinn Fein machine, acting as the party's office administrator at Stormont. In this role he was not in the tight inner circle of Gerry Adams and his "kitchen cabinet", but he was close to the heart of the republican political operation, and has been party to many decisions, and involvedin many conversations with important figures.
Republicans thought of him as utterly dependable, first of all in the IRA and then during Sinn Fein's period of political growth. The revelation of his double role as republican activist and as Special Branch informer shocked and horrified Sinn Fein and IRA people.
Republicans had known there were agents in the ranks, but it seems none had suspected Donaldson, partly because of his identification with the birth of the modern IRA and his association with Bobby Sands.
His unmasking came about in the most bizarre circumstances in October 2002. Following televised high-profile raids at Stormont, police arrested him and others, charging him with being part of a republican spy ring. The allegation against him was that he had been at the centre of an IRA operation which had amassed large amounts of confidential documents from both the Northern Ireland Office and from local political parties. The raids and arrests in effect brought down the power-sharing administration, with David Trimble, who was Ulster Unionist leader at the time, declaring that it could not function in the light of such behaviour. The Stormont Assembly has been suspended ever since.
In political terms, the Donaldson killing could not have come at a worse time. Tony Blair is due to launch an initiative today aimed at reviving the Assembly.
One of the ironies in this saga is that the IRA instructed him, a police informer, to start spying at Stormont, co-ordinating the collection and photocopying of confidential documents. That is presumably exactly what he had been doing for British intelligence: collating documents and information on the activities of Sinn Fein and the IRA, and passing them to police.
The IRA, in other words, ordered the British spy to start spying on the British - a tribute to his skill in avoiding suspicion.
There is no definitive answer to the question of why, in arresting and charging Donaldson, the police should have moved against their own agent of 20 years' standing. The most likely explanation is that he was selective in what he passed to his handlers. Security sources say he did not inform the police that the IRA had appointed him as its spy master at Stormont, and thus forfeited any legal protections he might have had. In all probability, they suggest, he withheld the information to protect himself and his family.
Evading detection for two decades was a major achievement on his part, and during his long experience he doubtless worked out many tricks and techniques for his own self-preservation. He probably became adept at handling his handlers.
Security sources also say his importance decreased over the years, an assessment which may help explain why Special Branch was prepared to sacrifice him. It may also be the case that the police and other agencies have other agents implanted at even more strategic levels within Sinn Fein and the IRA.
It is certain that they continue to use technological means of surveillance, making use of ingenious bugging devices planted in or near homes and other premises used by republicans. These can look like old rafters in a house, or when planted outside resemble pieces of rotting wood. Several years ago, a car used by Sinn Fein leaders Gerry Adams and Martin McGuinness was found to have fitted with a bug which broadcast their conversations to a satellite. In any event, Donaldson was clearly just one of a range of sources which the security forces used to maintain surveillance on republicans.
His unmasking presented the IRA with a huge dilemma, since its traditional punishment for informers is usually death. During the Troubles it killed at least 50 people, who it claimed were informers. But the Donaldson disclosure came after the IRA formally declared that it was abandoning violence.
In another irony, the republican used to announce this to the grass roots was Seanna Walsh, a friend of Donaldson's for decades.
Killing him would have made a mockery of what republicans saw as solemn and historic pronouncements that their movement had entered an entirely new phase of development, in which politics would displace terrorism.
The old procedure would have been to dispatch Donaldson with a bullet in the back of the head; instead, under the new dispensation, he was allowed to go free. The general assumption, which he himself shared, was that he was in disgrace, but not in danger of death.
Had he left Ireland he would probably still be alive: other informers have been spirited away to start new lives abroad. By staying in Ireland, Donaldson sealed his own fate, since there is nowhere that anyone can completely hide themselves away. Sooner or later, word would have got round about where the infamous informer was holed up.
His decision to stay was a fatal miscalculation: Ireland may be on the brink of a new and more peaceful era, but someone was determined that Donaldson would not live to see it.
How the IRA dealt with those it considered informers
Shot in 1993 by the IRA, who said he was a member of the organisation and claimed he was an informer. His body was found with gunshot wounds to the head.
IRA member from Londonderry was killed in 1990, his body found hooded and gagged on a border road. He had been missing from home for seven weeks. His mother said: "At the end of the day it's people like me, and their families, that are left to pick up the pieces."
Pensioner was shot six times at his west Belfast home in 1980. The IRA claimed he had given information to police about an attack in which a police officer was killed. A detective told the inquest there was no truth in the claim. The coroner described him as "a recluse causing no trouble to anyone."
Body was found on the border in 1986. Originally from Londonderry, he moved to England after an arms find, but later returned to the city. The Republican leader Martin McGuinness strongly denied claims by his mother that he helped persuade her son to return home, assuring him he would be safe.
Mother of three from Belfast was shot and her body left on the border in 1994. The IRA claimed she had been working as a police informer.
Former member of the IRA, his body was found close to the border in 1987. The IRA claimed he had worked for eight years as an informer with police in the Irish Republic, which his family denied.