The report was made to assistant commissioner Eugene Crowley by superintendent tom Curran in mid-1988.
Mr Curran was passing on the information he had received from his Northern Irish counterpart, superintendent Bob Buchanan.
Mr Buchanan and his colleague, chief superintendent Harry Breen, were killed several months by the IRA as they returned from a security meeting in Dundalk garda station.
The tribunal is examining allegations of garda collusion in the March 1989 IRA ambush.
Former assistant commissioner Mr Dermot Jennings said that Mr Crowley would have asked for a written report from Mt Curran, even if he had not believed the allegations.
“There isn’t anything that we can find or that has been made available to us,” tribunal barrister Ms Mary Laverty said.
Mr Jim O’Callaghan said his client, retired detective sergeant Owen Corrigan, did not accept that there had been any meeting between Mr Crowley and superintendent Curran.
Judge Peter Smithwick said that other witnesses has spoken about Mr Curran in laudatory terms.
“Nobody has said that he was telling a lie about this or that he was mistaken,” the retired judge said.
Mr Jennings said that Dundalk detective sergeant Owen Corrigan “had a reputation that he was a tough man, tough on the IRA.”
And he said the sergeant would not have been allowed on a course in surveillance techniques he taught in 1988 if there had been any concerns about him.
“It was a very sensitive course, about the tradecraft and the methods we use” Mr Jennings said.
Mr Jennings also said he would be “absolutely astounded” if reports of collusion received within two years of the Breen-Buchanan murder were not passed to an internal garda inquiry launched following press claims of collusion.
Three separate reports that the IRA had a garda “mole” in Dundalk were received within two years of their deaths.
Collusion claims first emerged within hours of the deaths of the two men, and resurfaced again in 2000 following the publication of Bandit Country by journalist Toby Harnden.
Documents provided to the intelligence section in Garda HQ indicated a “named member” of the IRA “had a garda contact”, the tribunal has previously heard.
Mr Jennings said he was not stationed in Garda HQ when the reports of collusion were received.
A decade later, he was the chief superintendent in Crime & Security branch, and one of his inspectors, Peter Kirwan, acted as a liaison between the branch and chief superintendent Seam Camon, who prepared a report on collusion allegations.
Mr Camon’s report concluded there was no evidence of collusion.
Mr Jennings said that surveillance operations had been mounted against the senior IRA figure named in the intelligence reports for several years, but the garda did not know the identity of any alleged IRA mole, including “technical surveillance” and “interception”.
He said Garda surveillance “could have identified this so called contact this man had, or identified other contacts the man had and eventually led on to the member of the garda referred to.”
“That would have been our goal to find out who this mole was.”
“If there was a wrong ‘un in the organisation, we would stop at nothing to root that person out”, Mr Jennings said.
Mr Jennings told the chairman that garda detectives did not carry firearms when in Northern Ireland, just as the two RUC men had been unarmed when they called to Dundalk garda station in 1989. The only exception was for bodyguard details escorting VIPs, such as when the president visited Belfast.