Members of the IRA involved in the murder of two RUC officers are refusing to give oral evidence to the tribunal investigating collusion in the killings.
Despite promises from the Smithwick Tribunal to protect their identities, the Provos refused to give oral evidence.
The tribunal, chaired by Judge Peter Smithwick, is looking into claims gardai gave information to the IRA leading to the ambush and murder of RUC officers, Chief Superintendent Harry Breen and Superintendent Bob Buchanan.
The tribunal is due to sit today to record what the IRA members told it in private meetings.
In his latest interim report to the Dail, Judge Smithwick outlined the setback, saying he was "very disappointed".
He said the tribunal's contacts with the former IRA member has reached a "definitive conclusion".
"Unfortunately, no former member of the PIRA is prepared to give oral evidence to the tribunal, notwithstanding extensive safeguards which the tribunal proposed to protect his or her identity and security," he wrote.