The now retired assistant commissioner, identified as Witness 27, travelled south to meet the source, and introduced him to ex-detective sergeant Owen Corrigan, the Smithwick tribunal heard.
On one occasion as the two men travelled to met the source, Corrigan spotted cars belonging to local republicans and warned Witness 27 they had to get away from the area quickly. Corrigan then escorted the senior officer to a Garda station and back across the border.
The witness said Corrigan saved his life by his actions.
The assistant commissioner said his superiors were aware of his meetings with Corrigan, the last of which took place less than a month before the IRA ambush in which RUC chief superintendent Harry Breen and superintendent Bob Buchanan perished on their way back from a meeting in Dundalk Garda station.
The tribunal is looking at claims that a Garda “mole” leaked information to the IRA about the visit, leading to their murder. Mr Corrigan has described allegations that he was behind any leak as “a monstrous lie”.
Witness 27 said he had reliable information that up to 30 IRA members were involved in the lethal ambush.
And he said the IRA obtained valuable intelligence as a result of the ambush.
On the night of the ambush, Witness 27 received five telephone calls asking “Who is this?” and although he changed his ex-directory number, the IRA had his new number, home address and details of his car within 24 hours.
A sixth attempt was made on his life a short time later, and as a result he moved to England,
The witness broke down as he spoke about the news of the murders coming through on a telex machine. At first he thought it was a report of two IRA men killed by their own people, but realised the truth when he recognised the registration number of superintendent Buchanan’s car. He said he too had been due to travel to Dundalk, but a last minute meeting in Northern Ireland led to a change of plans.
He said he had opposed the planned operation against Thomas “Slab” Murphy which led to the cross-border meeting, as there was insufficient evidence to go ahead, but Northern Ireland secretary Tom King was adamant it should proceed, banging a table to make his point.
And he added that he never had any doubts about security in Dundalk garda station, saying the IRA could have arranged the ambush at 30 minutes notice.
Earlier, the tribunal was told by ex-chief superintendent John Nolan that detective sergeant Corrigan was suspected of involvement in cross-border smuggling and barred from entering Drogheda garda station.
Mr Nolan said he did not believe the detective sergeant â€“ or any other garda in Dundalk – was an IRA mole, despite other disciplinary problems, and said an inquiry led by assistant commissioner Edward O’Dea in the wake of the murders of the two RUC men was not designed to rule out a “mole”.
And he said there was insufficient evidence the detective was involved in smuggling despite strong suspicions.
Barrister Jim O’Callaghan, representing Mr Corrigan, said there was “no positive evidence” his client was involved in smuggling.
Chief superintendent Nolan said the Provisional IRA would have to sanction any cross-border smuggling operation.
In an internal report on Mr Corrigan, the senior officer wrote about the “adverse effect his conduct is having on morale and discipline generally.”
On two occasions a recommendation was made to transfer the detective to Dublin, but the decisions were appealed.
And Nolan said he opposed a suggestion made at a review board that the detective should be transferred from Dundalk to Drogheda.
He said there was a “long history of members frustrating inquiries” in the force.
Mr. Corrigan went on sick leave before he could be transferred.