The Irish government has agreed a co-operation arrangement with the Omagh Bombing Inquiry.
A Memorandum of Understanding (MoU), which had been expected, was signed off at a cabinet meeting in Dublin.
Twenty-nine people, including a woman expecting twins, died in the 1998 Real IRA attack, which was carried out from the Irish Republic.
The MoU will allow the inquiry access to material held by the Irish government.
The inquiry secretary, Sam Hartley, welcomed the agreement.
"This formal agreement marks a significant step forward in allowing the inquiry access to material, information and assistance from the government and agencies in Ireland," he said.
"Agreements of this nature are not commonplace in inquiries.
"On behalf of the inquiry, I express my gratitude to the government of Ireland for its repeated commitment to assist the inquiry."
Some victims' relatives, however, had wanted Dublin to establish a parallel inquiry in its own jurisdiction.
They believe it is the only way to ensure full disclosure of information and compel witnesses based in Ireland to provide evidence.
What have the Omagh families said?
Speaking to the BBC's Evening Extra programme, Michael Gallagher, whose son Aiden died in the bombing, welcomed the Irish government engagement.
"But we need more detail and we need to know in practical terms how this is going to work out," he said.
That the Irish government did not meet the Omagh families while working on the MoU, Mr Gallagher said, was "a huge opportunity missed", adding there were a number of areas within the memorandum that gave cause for concern.
Those concerns, he said, included whether witnesses would give evidence under oath and in which jurisdiction witnesses would give their evidence.
Further detail, Mr Gallagher said, was also needed on how the evidence of gardaí (Irish police) and "intelligence witnesses" would be managed.
He said families should now meet the Irish government to talk through some of those issues.
