The Smithwick Tribunal has been told that there were between 300-400 members of the Provisional IRA and other armed republican groups in Dundalk in 1989.
The information came from former Special Branch Detective Joe Flanagan.
He said their names were 'listed' by the gardaí, which meant they noted details like what these people were up to, who they were meeting and their cars.
Mr Flanagan said he also believed another former garda, Finbarr Hickey, when he said he had been asked by then Det Sgt Leo Colton to sign blank passport forms.
Those forms ended up in the hands of active IRA members. While Hickey went to prison for the offence, Mr Colton was never charged. Mr Colton has always denied the allegation.
The Smithwick Tribunal is investigating whether Mr Hickey, Mr Colton or another former Det Sgt Owen Corrigan passed information to the IRA, which led to the murders of two senior RUC officers.
Chief Supt Harry Breen and Supt Bob Buchanan were killed in March 1989.
An earlier witness, retired detective Jim Lane also told the tribunal that he believed Mr Hickey when he said he had been asked by Mr Colton to sign the forms.
Mr Lane said he had known Mr Hickey from when he was a child and he believed what he had said.
Under cross-examination both witnesses said Dundalk was a dangerous place to be a detective during the Troubles.
Mr Flanagan said he had been shot at by the IRA on one occasion.
Both men also agreed that Mr Corrigan had been on the frontline of the fight against the IRA.
Both witnesses worked in the same detective unit based in Dundalk Garda Station, which was led by Mr Corrigan.
Mr Flanagan also told the tribunal that he met Mr Corrigan after Mr Corrigan had been kidnapped and beaten up in 1996, but he never said who had abducted him.
