The Smithwick Tribunal will today hear evidence from Michael McKevitt, who is serving a 20-year sentence for directing terrorism and membership of the Real IRA.
The tribunal will sit at the Criminal Courts of Justice to facilitate his evidence.
McKevitt is one of four people found liable by a civil court in Belfast for the Omagh bombing.
He will travel from Portlaoise for the hearing.
The tribunal is inquiring into allegations that members of the Gardaí colluded with the IRA in the killings of two senior RUC officers, chief superintendent Harry Breen and superintendent Bob Buchanan in 1989.