A former British double agent in the IRA has denied being a pathological liar over claims a suspected Garda mole destroyed evidence from one of the worst atrocities in the Troubles.

Peter Keeley — also known as Kevin Fulton — told a tribunal into alleged Garda-IRA collusion that he heard a detective got rid of vital information after the 1979 Narrow Water bomb attack, which killed 18 British soldiers.

The agent, who spied on the terror group from the 1980s, has claimed Dundalk-based Detective Garda Sergeant Owen Corrigan assisted volunteers on several occasions over the years.

Mr Corrigan has strenuously denied the allegations of collusion, which he has called a monstrous lie.

The Smithwick Tribunal is investigating allegations of Garda collusion over the IRA murders of senior RUC officers, Chief Superintendent Harry Breen and Superintendent Bob Buchanan on the Irish border, minutes after a Garda meeting.

Mr Keeley, a Newry-born Catholic who joined the British Army in 1979, was giving evidence for a second day in Dublin behind a screen.

The 51-year-old again claimed Mr Corrigan had tipped the IRA off that Mr Breen and Buchanan were in Dundalk the day they were ambushed, March 20 1989.

He also said the detective had cleaned fingerprints from where a 1,000lb bomb was found in Omeath and had told volunteers Co Louth farmer Tom Oliver was an informer. Two months later, in July 1991, Mr Oliver was kidnapped and murdered.

Under cross-examination, Mr Corrigan's barrister, Jim O’Callaghan, said he could prove Mr Keeley was a pathological liar.

Mr Keeley said he had told his handlers Mr Corrigan was an IRA mole, but that he also knew of RUC officers giving information to units. Mr O'Callaghan told Mr Keeley several former police officers — ex-members of the RUC/PSNI, of security forces and of An Garda Siochana — had questioned the agent's credibility, calling him a liar and fantasist.

“I have done things that I'm not proud of. Things my handlers know I have done and I'm party to that,” he replied.

“Maybe it's because if I go down the road, they're coming with me.

“Maybe it's good to discredit people who can do them harm.”