Evidence shows Owen Corrigan passed information to the IRA – but it cannot be proved he colluded in the murders of Harry Breen and Bob Buchanan, the tribunal found.
The Smithwick Tribunal dismissed the former detective’s evidence to the long-running inquiry as untruthful, vague, evasive and inconsistent.
Judge Peter Smithwick concluded it is impossible to attach any credibility to Mr Corrigan’s testimony when he frequently gave completely different answers to the same question.
But despite evidence he had an inappropriate relationship with subversives, it could not be proved beyond doubt that he colluded in the double shooting.
It was the naming of Mr Corrigan as an IRA mole that sparked the eight-year probe into Garda collusion with the Provos.
In April 2000, DUP MP Jeffrey Donaldson stood in the House of Commons and, under parliamentary privilege, said the detective sergeant colluded with Provos in the 1989 murders.
Mr Corrigan denies the allegations, made to the MP by British double agent Peter Keeley, also known as Kevin Fulton.
Mr Corrigan joined the force in September 1960 and was stationed in Drogheda, Co Louth, a year later. He spent most of the next 30 years stationed along the border.
He was in work the day the Mr Breen and Mr Buchanan were killed, but claims he never saw them at the station and had no involvement with their killing.
Fulton claimed “a friend’’ of the IRA – who he alleged was Mr Corrigan – tipped off the IRA that the RUC were in Dundalk on the day they were ambushed.
Mr Corrigan has called the claims a monstrous lie.