[3] By the late 1960s, the teenaged O'Callaghan had ceased practising the Catholic faith, adopted atheism and had become interested in the theories of Marxist revolutionary politics, which found an outlet of practical expression in the sectarian social unrest in Northern Ireland at that time, centred on the activities of the Northern Ireland Civil Rights Association.
In 1969, an outbreak of communal violence broke out in Northern Ireland and believing that British imperialism was responsible, O'Callaghan joined the newly founded Provisional IRA at the age of 17.
Soon afterwards, he was arrested by local Gardaí after he accidentally detonated a small amount of explosives, which caused damage to the homes of his parents and their neighbours.
[8] On 23 August 1974 O'Callaghan killed Detective Inspector Peter Augustine Flanagan, a 47-year-old Catholic police officer and head of the Omagh Division of the Royal Ulster Constabulary's Special Branch, by shooting him repeatedly with a handgun in a two-man IRA attack in a public house in the town of Omagh in County Tyrone.
[20] In 1984 he notified the Garda of an attempt to smuggle seven tons of AK-47 assault rifles from the United States to Ireland aboard a fishing trawler named Valhalla.
[citation needed] The shipment had been organised by the Winter Hill Gang, an Irish-American crime family of South Boston, Massachusetts.
As a result of O'Callaghan's warning, a combined force of the Irish Navy and Gardaí intercepted the boat that received the weaponry, and the guns were seized.
[22] O'Callaghan claimed to have been tasked in 1983 by the IRA with placing 25lb of Frangex in the Dominion Theatre in London,[23] to try to kill Prince Charles and Princess Diana who were due to attend a charity pop music concert there.
[citation needed] After becoming disillusioned with his work with the Irish Government after the murder of another of its agents within the IRA (Sean Corcoran in County Kerry in 1985), which it had failed to prevent despite O'Callaghan's warnings of the threat to him, and sensing a growing threat to himself from the organisation which had become suspicious of his own behaviour, O'Callaghan withdrew from the IRA and left Ireland to live in England, taking his wife and children with him.
[25] Although the Royal Ulster Constabulary offered him witness protection as part of the informer policy, O'Callaghan refused it,[citation needed] and was prosecuted under charges of two murders and 40 other crimes, to all of which he pled guilty, committed in British jurisdiction with the IRA.
In 1998 O'Callaghan published an autobiographical account of his experiences in Irish Republican paramilitarism, entitled The Informer: The True Life Story of One Man's War on Terrorism (1998).
[26]In 2002 he was admitted to The Nightingale Hospital, Marylebone, an addiction and rehab center where he underwent a rehabilitation program for alcohol dependency.
[29] O'Callaghan died by drowning after suffering a heart attack at the age of 63 while in a swimming pool in Kingston, Jamaica in August 2017, while visiting his daughter, Tara.
A 1997 article in An Phoblacht alleged that O'Callaghan "has been forced to overstate his importance in the IRA, and to make increasingly outlandish accusations against individual republicans".
[32] O'Callaghan also claimed to have attended an IRA finance meeting in Letterkenny in 1980 with, among others, lawyer Pat Finucane and Sinn Féin politician Gerry Adams.
The RUC and the Stevens Report indicated that, to their knowledge he was not believed to have been an IRA member, although he came from an Irish Republican background and three of his brothers (Dermot, John, and Seamus) had been involved in paramilitarism.