Gardaí and others should testify at the Omagh Bombing Inquiry if requested, Taoiseach Micheál Martin has said.
The Taoiseach said it is important that Ireland is "open and transparent" and urged people to co-operate with the probe.
The inquiry is examining whether the Real IRA bomb attack in 1998 could have been prevented.
The Omagh bomb, which killed 29 people including a woman who was pregnant with twins, is the worst single atrocity in the Troubles in Northern Ireland.
The bomb was prepared in the Republic and transported by the Real IRA members who then fled back across the border after planting the bomb in the centre of Omagh on a Saturday afternoon.
After months of negotiations between inquiry staff and officials, a Memorandum of Understanding (MoU) was announced yesterday.
The MoU is to allow the inquiry chairman Alan Turnbull and members of his team to access material held by the State.
Mr Martin welcomed the MoU and said Ireland must do all it can to ensure "the fullest information" is made available to the inquiry.
Asked about reports of retired gardaí being advised not to testify, the Taoiseach told Newstalk radio: "I regret that if that's the case.
"I think people should testify and people should co-operate, in my view, with the inquiry.
"I take the view that those who planted that bomb are the people who are culpable for the appalling loss of life that arose from the planting of that bomb in Omagh.
"We all remember it, it's a day that will always be etched in our minds."
He said the bomb "was made in the Republic", adding that many people connected "were pursued on different charges and on different issues".
Mr Martin said the "key issue at stake" was whether security forces were aware of the attack and did not intervene.
He said: "That's why an inquiry needs to be comprehensive to deal with that assertion, and that's why it's also important - because things get said about the Republic from certain quarters - it's important that we here are very open and transparent in respect of these issues."
The inquiry said in a statement that it will "continue to discuss further cooperation with the Government of Ireland, such as taking evidence from witnesses from the Republic of Ireland".
The Minister for Justice said yesterday that legislation may be needed for witnesses such as gardaí to appear before the inquiry.
Jim O'Callaghan also said that "security sensitive" information will be redacted but that the inquiry will be able to review unredacted parts in person, if required.
"It's my intention and the intention of Government to ensure that witnesses from Ireland, be they gardaí or former civil servants, or anyone with information, to provide evidence that can be used by the inquiry," he said.
All those who were killed were remembered during commemorative hearings at the inquiry earlier this year and evidence was also heard from those who were injured and emergency workers who responded.
The next phase of public hearings, due to start from June, will hear from core participants, including the Police Service of Northern Ireland and former chief constable Ronnie Flanagan.
