Up to 400 suspected IRA members lived in the Dundalk area in the late 1980s, a retired detective garda told the Smithwick tribunal.
The inquiry is examining claims of Garda collusion in the murder of two RUC Officers killed in an ambush as they returned from a meeting in Dundalk on 21 March 1989.
Chief Superintendent Harry Breen and Superintendent Bob Buchanan were two two most senior RUC officers killed during the Troubles.
Retired detective Joe Flanagan said that between three and four hundred suspected subversives were listed in Garda files as living in Dundalk at the time of the killings.
“It means we believed they were members of the IRA. And that would include members of the IRA, INLA, other organisations,” he said.
Members of the detective branch monitored the movements of listed individuals, which were correlated by collators in the station and in Garda HQ in Dublin.
Mr Flanagan said he was detailed to search the home of a retired garda sergeant, Leo Colton, following an investigation into forged passports a decade after the attack.
Former sergeant Finbar Hickey was convicted in 2001 of forging passports some of which were found in the possession of IRA members.
Hickey told detectives he had made passport applications at the behest of Mr Colton, who was never charged with any offence.
“It’s my opinion that Mr Hickey was telling the truth,” Mr Flanagan said.
But he said he did not know of any association between either man and known republicans.
“Finbar Hickey was very easy led and he would have been friendly with Leo Colton,” Mr Flanagan said.
The witness also said it was “shameful” that retired detective sergeant Owen Corrigan had been identified in media reports as a Garda “mole”.
“I never actually believed there was any substance at all to those reports,” another witness, retired detective garda Jim Lane said.
Both Mr Lane and Mr Flanagan had worked in the detective branch, where Mr Corrigan also worked.
Mr Lane said he had known detective sergeant Corrigan “a lifetime”.
“I know he was kidnapped and I know he got a bad beating. I visited him in hospital and he certainly got a very bad beating,” he said, referring to an incident in the mid-1990s.
Mr Flanagan said that as he worked in the detective branch he did not know retired sergeant Leo Colton, as Colton was a uniformed sergeant.
Garda Michael Johnson worked in the communications room in Dundalk station, and said he had no recollection of the day, although his diary showed he had worked a morning shift.
Detective Garda Errol Boyle told the tribunal he was on patrol most of the day, and did not know that two RUC officers had visited the station until after the ambush, when he got a radio call to return to the station.
The tribunal resumes on Tuesday 29 June.