The organisation representing retired gardaí has written to the incoming Garda Commissioner Drew Harris expressing concern regarding his testimony at the Smithwick Tribunal six years ago.
The Garda Síochána Retired Members Association, which represents almost 6,000 retired gardaí and widows, said former gardaí particularly in Dundalk had "very real concerns" about his evidence.
The association says because of Mr Harris' "key role with MI5 and Special Branch Intelligence, he must hold sensitive information of interest to the Irish State."
It has questioned whether he will share this information in his role as Garda Commissioner.
The Association also claims that Mr Harris "has been swamped in controversy before he even gets off the blocks."
The 53-year-old former Deputy Chief Constable of the PSNI is due to be sworn in as Garda Commissioner just after midnight on Monday morning.
The Smithwick Tribunal found there was collusion between members of the gardaí and the IRA in the murders of two senior RUC officers and that "on the balance of probability" gardaí at Dundalk were most likely involved.
The former Garda Commissioner Martin Callinan rejected the finding of collusion.
Mr Harris' evidence was described by counsel for the gardaí at the Tribunal as "nonsense on stilts".
