Mr Corrigan, one of three former garda sergeants granted legal representation before the tribunal, denies allegations of collusion against him and as described them as a “monstrous lie”.
And Corrigan’s lawyers said a “C6” rating on the report showed that while the informant was considered reliable, the information was not really believable.
The tribunal, now in its final weeks, was set up to look at allegations of Garda collusion in the March 1989 deaths of two senior RUC officers, chief superintendent Harry Breen and superintendent Bob Buchanan.
Witness Z, a retired RUC special branch officer, told the tribunal that John McAnulty was the source of information he recorded in a 1985 report stating Corrigan was the source of information on RUC and Garda movements.
The report by Witness Z â€“ known as an SB50 â€“ has been referred to several times at the inquiry, but until now no witness has been able to give evidence about its source. The report was one of the critical pieces of evidence which led Canadian judge Peter Cory to recommend a tribunal into the Breen/Buchanan killings.
Witness Z said that McAnulty was credible and believable, and told him that Corrigan was passing information to the IRA.
Lawyers clashed heatedly before retired district court president Peter Smithwick over whether Mr McAnulty, who was shot dead in 1989, should be identified.
PSNI barrister Mark Robinson protested that naming an informant could have a “chilling effect” and would “seriously damage the flow of information” about terrorist activities.
And he said it was “outrageous” that the tribunal maintained “radio silence” when he tried to find out what evidence Witness Z would give.
Tribunal barrister Ms Mary Laverty SC said Mr McAnulty was kidnapped, tortured and killed by the IRA in 1989.
Mr Robinson objected again when the witness was asked about the grading of information received informants for reliability
“There must be a limit to the amount of things the PSNI can cover up and sweep under the carpet,” judge Peter Smithwick told the PSNI barrister.
Witness Z told the tribunal he wrote the SB50 report on Corrigan in 1985. He said that McAnulty was his informant for 17 years by his death in 1989.
The witness said he decided to come forward with his evidence after he was told sergeant Corrigan was absent from work without leave the night McAnulty died by the Breen family’s solicitor John McBurney. He said he decided to give evidence “for the sake of justice and to get this out of my system.”
Tribunal barrister Ms Mary Laverty SC said the evidence of former garda chief superintendent John Nolan was that Mr Corrigan was on duty that night, but could not be found anywhere.
The witness said e was not accusing Mr Corrigan of any involvement, and had no evidence to suggest otherwise, but said the former detective sergeant needed to give a reason for where he was that night.
And he said he had no evidence linking Mr Corrigan to the deaths of chief superintendent Breen and superintendent Buchanan.
And he told garda lawyers the SB50 report “fully reflected what he [McAnulty] told me, what he meant to say, it may not be in the words he spoke but certainly it was what he inferred, what he said.”
Michael Durack SC, representing the garda commissioner, had put it to th witness that the memo was written in “police speak”, and not a verbatim account.
Witness Z also said than on an occasion when he met with Garda inspector Dan Prenty in Dundalk garda station, the inspector signalled him to stop talking when sergeant Corrigan entered the room.
And he said that shortly after he was assigned to the special branch in Newry, an RUC colleague warned him to be careful of sergeant Corrigan.
Barrister Jim O’Callaghan Corrigan said that Mr Corrigan, who wasscheduled to give evidence next week, was ill and might not be able to appear. The judge said that only two witnesses remained to give evidence.
The tribunal resumes on Tuesday.