Relatives of those killed in the "Real" IRA bombing of Omagh in 1998 are stepping up calls for a public inquiry.
Yesterday was the 6th anniversary of the bombing that killed 29 people, including a mother pregnant with twins, and injured hundreds.
The Minister for Transport, Mr Brennan, attended a service in Omagh yesterday to mark the anniversary.
Mr Brennan represented the Government and the Taoiseach, Mr Ahern, at the service in the town's Memorial Garden.
Relatives pressed the North's Human Rights Commission to support a public inquiry having met the organisation last week.
Mr Michael Gallagher, whose son Aidan died in the attack, said a full cross-Border inquiry into the events of that day was needed as soon as possible.
He told the BBC: "We just feel that coming up to the 6th anniversary we have had I think six inquiries in the last six years and at the end of that process the families know very little. We feel that the best way to know what happened at Omagh on the 15th of August was to have a full cross-Border public inquiry."
The Omagh bomb was the single worst atrocity in 30 years of violence in Northern Ireland. Only Colm Murphy a publican and businessman from Dundalk Co Louth, has so far been jailed in connection with the bombing. In January 2002 the Special Criminal Court in Dublin jailed him for directing terrorism.