The Smithwick Tribunal into allegations that gardaí colluded with the IRA in the murder of two senior RUC officers is to begin hearings in June.
The inquiry, which is expected to hear five months of evidence, has written to legal teams setting the opening date for Tuesday June 7.
Chaired by Judge Peter Smithwick, it is examining allegations that gardaí colluded with the IRA in the killing of Chief Superintendent Harry Breen and Superintendent Bob Buchanan, murdered in an IRA ambush on March 20 1989.
The tribunal was set up in Dublin in 2005.
The inquiry will also inquire if other State employees played any part in the alleged collusion.
The officers were returning from a meeting with a senior Garda in Dundalk when they were ambushed and killed just north of the border in Co Armagh. The meeting in the garda station was arranged only on the morning of the killings.
The Provisional IRA claimed responsibility for the murders.
Tribunal lawyers have also written to legal teams asking them to attend a sitting on May 17 to make applications for further representations for people or organisations the inquiry may involve.