Denis Donaldson, who was brutally murdered on Tuesday aged 56, was, variously, an IRA volunteer, a republican hunger striker, and a central figure in Sinn Fein’s “international department” before becoming its senior administrator; he was also a British double agent for more than 20 years.

His exposure came as a shattering surprise because of his impeccable republican credentials as a “pre-69er” – someone who, like Gerry Adams, had joined the movement before “the Troubles” broke out.

Donaldson’s career started to unravel with his arrest in 2002. The police accused him of being a key figure in a Sinn Fein spy ring, an event which led to the fall of both the power-sharing executive and David Trimble, the Ulster Unionist leader.

Three years later the Director of Public Prosecutions announced that going ahead with a court case was “no longer in the public interest,” and soon after Donaldson learned that he was to be exposed. But instead of agreeing to be spirited abroad by the security services, he appeared on RTE television in Dublin, explaining that he had been recruited “after compromising myself during a vulnerable time of my life.” This may have been a reference to his taste for the ladies; he was known as “a chaser” in Belfast parlance.

Even that experienced watcher of events north of the border, the Taoiseach (prime minister) Bertie Ahern, admitted that the confession was “as bizarre as it gets.”

Denis Martyn Donaldson was born in 1950 into a republican family in the Catholic Short Strand enclave of east Belfast. He earned a place in the republican hierarchy for his role in the socalled “Battle of St Matthews” of June 1970, a street protest against local Protestants that was regarded as one of the IRA’s seminal moments. Next he was imprisoned for four years for his exertions as a “shop-lifter,” his droll way of describing his blowing up of shops with explosives.

In the Maze prison he took part in the hunger strike that secured “political status” for IRA prisoners, and was photographed there with Bobby Sands, who later starved himself to death.

Donaldson earned a mention in Gerry Adams’s memoir, Hope and History, as a man “with ideas and energy.” But he mostly remained behind the scenes, with a reputation for getting things done.

It was in the early 1980s that he started to work for the British, providing them with the IRA’s foreign contacts.

As a member of Sinn Fein’s “international department” he was arrested at Orly when returning on a false passport from Lebanon in 1981. After the authorities released him he established himself as a “quasi-ambassador” to the Palestine Liberation Organization and the Basque group Eta. He also tried to obtain the release of the hostage Brian Keenan, who declared after Donaldson’s arrest in 2002: “Two human beings put their lives at risk on my behalf. One was Terry Waite and the other was Denis Donaldson.”

Donaldson was sent to America to secure support for Sinn Fein’s surrogate organization, NORAID, which was being jeopardized by a rival group that disagreed with the increasing weight being placed on “politics” instead of “armed struggle.”

As head of Sinn Fein’s administrative team at the Northern Irish Assembly from 1998 he seemed a conventional republican. Nothing in his conversation suggested that his antipathy for the British was an act.

His death came just before the Irish news magazine Magill was to name him its “survivor of the year.”