Within hours of giving evidence in a high-profile libel action, O'Callaghan was arrested - then released - by Irish police over allegations that he was involved in a murder.
A colourful character, O'Callaghan claimed to have given tip-offs that averted IRA murders and bombings. Today, despite becoming a vocal critic of the IRA leadership, his shadowy role is raising morally difficult questions about whether authorities put secret intelligence before saving human life.
His past also raises some uncomfortable questions for the Daily Telegraph, which lastweek argued against the serialisation of the story of Mary Bell, saying it was wrong that a killer should profit from their story. Yet the newspaper has now reportedly agreed to publish excerpts from his memoirs.
O'Callaghan pleaded guilty in 1990 to two murders and has spoken publicly of his role in a third, about which he was questioned last week.
Now 42, the Kerry-born Provo walked into a police station in 1988 and confessed to killing a UDR soldier, Eva Martin, during an attack on RUC barracks in Tyrone in May 1974, and an RUC officer, Peter Flanagan, shot in an Omagh pub in August that year. He has also admitted that, while acting as a garda mole, he shot dead an alleged Cork IRA informer, John Corcoran, in 1985.
Despite his admissions, there have been scant moves to date to bring the case to court. Last week in Dublin's High Court, O'Callaghan was again holding centre stage in his campaign against his former republican comrades. He testified that Sinn Fein leaders - including Gerry Adams and Martin McGuinness - attended a gathering of the IRA's "revolutionary council" in 1983.
O'Callaghan was a somewhat nervous witness for the Sunday Times in its appeal against a libel award to Thomas "Slab" Murphy. In 1985 the paper named Murphy as IRA director of operations in Northern Ireland.Reuse content