IRA informer turned author Sean O'Callaghan has died aged 63, a close friend has said.
It is understood the convicted republican killer died while swimming in a pool in Jamaica.
The Co Kerry native was in the Caribbean visiting his daughter.
O'Callaghan detailed the inner workings of the IRA in his best-selling book, The Informer, in 1999.
Among his revelations was the claim he thwarted a plot to murder the Prince of Wales and Princess Diana in the 1980s.
O'Callaghan was jailed in the 1990s after walking into a police station in England and admitting two IRA murders. He was released in 1997.
He lived under the threat of reprisal from former IRA associates ever since he went public about his life as an informer.
Senior republicans dismissed O'Callaghan as a Walter Mitty character whose claims were embellished or untrue.
O'Callaghan joined the IRA in the mid-1970s. By the end of the decade, he had turned informer and started providing information to the Irish Garda.
He was close friends with historian and author Ruth Dudley Edwards. She paid tribute.
"It's beginning to hit the news that Sean O'Callaghan, the IRA killer who became an unpaid spy for the Gardai, has died," she said.
"He drowned yesterday while swimming in a pool in Jamaica, where he was visiting his daughter.
"He was a man of exceptional ability and courage, and he spent most of his life finding ways of atoning for the crimes he had committed before at 20 he realised he was fighting in a squalid sectarian war rather than a resistance movement.
"He and I were very close friends for more than 20 years.
"And, like all his friends, I loved him very much and owe him a great deal for his insights, his wise advice, the depth of his knowledge of politics, history and the human condition."
O'Callaghan supported relatives bereaved in the 1998 Real IRA Omagh bomb in taking a landmark civil case that resulted in four republicans being held liable for the outrage.
Michael Gallagher, whose son Aiden died in the bomb, passed on his condolences.
"My experience of Sean O'Callaghan was always a positive experience," he said.
"He did a great deal to help the Omagh families get some form of justice."
Earlier this year, O'Callaghan gave evidence, from a secret location, to a Belfast inquest examining the sectarian murder of 10 Protestant workmen near the South Armagh village of Kingsmill in 1976.
He named two senior IRA men as being responsible for the massacre.