Former detective sergeant Owen Corrigan told the tribunal that he was told that there was a threat to John McAnulty, a grain trader and smuggler, but did not warn him or the RUC.
Mr McAnulty was abducted and killed by the IRA in Co Louth in July 1989. His body was later found in Northern Ireland.
In a confidential report on the day of the abduction, Mr Corrigan wrote that Mr McAnulty had been abducted because he had been arrested the previous year during a smuggling investigation, but released without charge. As a result, he was suspected of informing by the IRA.
Tribunal barrister Mr Justin Dillon said this information could have saved Mr McAnulty’s life.
“It could have, it might have, yes,” Mr Corrigan said.
Mr McAnulty has also been identified at the tribunal as the source behind a 1985 RUC special branch report which said sergeant Corrigan was passing information to the IRA.
The tribunal is investigating allegations of garda collusion in the deaths of two RUC officers, chief superintendent Harry Breen and superintendent Bob Buchanan, as they returned from a security meeting in Dundalk Garda station on 20 March 1989.
Mr Corrigan denies allegations of collusion, and has described them as a “monstrous lie.”
Mr Corrigan said there had never been a complaint made against him by a member of the public about his conduct as a garda, and although senior officers north and south met regularly, his name was never mentioned at those meetings.
And he said allegations that he was passing information were “purely vindictive destruction of my character.”
Mr Corrigan, who was abducted and beaten up by the IRA in 1995, said he was questioned about garda sources during his ordeal.
And he said he “felt he was set up” to be kidnapped by another man, Mr Francis Tiernan, who was abducted at the same time when they met in Mr Tiernan’s car in a hotel car park.
“As soon as I sat in the back seat of the car, suddenly I found the door being opened,” Mr Corrigan said. “I was pulled out of the car and put in the back of a van.”
“I was blindfolded and beaten all the time,” he added. “They went through every incident of note that happened for years.”
Mr Corrigan had retired from An Garda Síochána in 1991, but he said there was a belief in some quarters that he was still working for Garda HQ.
“At least when I was in the Garda I had the backing of the Garda. When I came out I was very vulnerable,” he said.
One of Mr Corrigan’s last assignments before retirement was to meet with a “middleman” in a bid to recover the Beit art collection, stolen by Martin “the General” Cahill, at the request of then chief superintendent Noel Conroy, later garda commissioner.
“There was a collective decision of the cabinet that this man, the General, had to be put down,” Mr Corrigan said.
He said he went to the meeting although he was on sick leave at the time due to nervous exhaustion because his psychiatrist had told him “one way to improve my health would be to do something positive.”
And he added that Mr Conroy was “a man I respected and I was delighted to work for him.”
“You weren’t suffering from nervous exhaustion, it was a deception,” tribunal barrister Mr Justin Dillon SC put to Mr Corrigan.
“That’s a very serious allegation There has never been any suggestion before today that this man was engaged in a deception,” objected Mr Jim O’Callaghan SC, Mr Corrigan’s barrister.
“If the tribunal is going to question a medical diagnosis, it should do so on the basis of medical evidence.”
“I was fully certified for any sickness I had,” Mr Corrigan said. “If you have any doubt, as my counsel said, call the medical experts.”
“Isn’t it the case that mental experts can only go on what you tell them?” Mr Dillon asked.
“Can you go any further?” replied Mr Corrigan.