Owen Corrigan served as a detective sergeant in Dundalk from the 1970s to until 1990.
He told the Smithwick tribunal that one in four republicans was providing information to either the British Army or the RUC, “including the highest echelons.”
“They congregated at night in Dundalk and travelled up to Northern Ireland the following day,” Mr Corrigan said. “They were going up weekly to claim this money and they were telling on each other. They had no sense of loyalty.”
He said the British authorities paid “huge sums of money” for “tittle-tattle” and “useless information”.
The tribunal is investigating allegations of garda collusion in the deaths of two RUC officers, chief superintendent Harry Breen and superintendent Bob Buchanan, as they returned from a security meeting in Dundalk Garda station on 20 March 1989.
Mr Corrigan denies allegations of collusion, and has described them as a “monstrous lie.”
Mr Corrigan said he believed the attack in which Breen and Buchanan died was planned since the previous Christmas.
And he said he was told by a senior RUC officer, Brian Fitzsimons, that he did not trust Sunday Times journalist Chris Ryder, who he said worked for MI5. Mr Corrigan rejected evidence from Ryder that he offered the journalist stories for money.
Mr Corrigan said he was attempting to comply with tribunal requests for his accounts, but the banks would only provide him with his records for the last seven years, and were “most reluctant when I told them I wanted to go back to 1988.”
The former detective also dismissed an RUC special branch report which said he was passing information to the IRA as “tittle-tattle”. He said the senior officer who received the report knew it was untrue, as Corrigan shared information with him regularly.
“He made a decision, he had much more up-to-date information, I was keeping him posted,” Mr Corrigan said. “I was keeping the security forces up-to-date.”
And he said disciplinary proceedings against him by An Garda Síochána were “totally fabricated”, and one of the officers involved had come to him and apologised afterwards.