Witnesses to a tribunal investigating possible collusion between the IRA and Irish police in the murder of two RUC officers will be given immunity.
The Attorney General confirmed that information given by potential witnesses, including an IRA informer, cannot be used in a future prosecution.
Judge Smithwick's tribunal in Dublin will investigate the murder of Harry Breen and Robert Buchanan in 1989.
The judge asked the UK's attorney general to guarantee immunity.
"The purpose of any inquiry is to get to the truth of what happened and such inquiries usually only take place long after the possibility of any prosecutions based on evidence currently available has passed," said a spokeswoman for the attorney general.
"It was considered that that objective might be more difficult to achieve if witnesses were concerned that what they say may be used against them.
"The Irish statute that the tribunal is set up under provides a statutory undertaking along similar lines but that would not protect witnesses from potential prosecution in Northern Ireland or England and Wales.
"It was for that reason the judge approached the Attorney General."
RUC Chief Superintendent Harry Breen and Superintendent Robert Buchanan were murdered by the IRA in an ambush shortly after leaving a meeting in Dundalk Garda station on March 1989.
Claims of collusion between at least one garda and the IRA resulted in the Smithwick Tribunal being set up in May 2005.
However, several key witnesses including former members of the British security services and one man who claimed he worked undercover in the IRA have refused to cooperate fully with the Tribunal.
They had claimed that they risked prosecution by revealing what they had done and for breaching the Official Secrets Act.
The inquiry is one of six recommended by Judge Peter Cory in 2004.
"The Attorney General was asked, and provided, an undertaking in identical terms to the Hamill and Nelson Inquiries," said the spokeswoman for the Attorney General.
"The Wright Inquiry did not ask for one. The provision of such an undertaking is not unusual. One was also provided for the Bloody Sunday Inquiry.
"There is nothing secretive about this. The Hamill and Nelson Inquiries publish the undertaking on their websites and notify witnesses of it."