The Omagh Bombing Inquiry is seeking “clear written agreements” with the Irish Government over co-operation.
This would be to include access to materials held in the Republic and witnesses who live there.
The first public hearing of the inquiry took place on Tuesday at the Strule Arts Centre in Omagh.
It is probing whether the dissident republican bombing which devastated the Co Tyrone town, killing 29 people, including a woman pregnant with twins, could have been prevented.
It has been contended that the terrorists prepared the bomb in the Republic and also fled back across the Border after planting it in Omagh on August 15th, 1998.
The inquiry into the atrocity was ordered last year by former northern secretary Chris Heaton-Harris following a legal challenge by a bereaved family.
Belfast High Court judge Mr Justice Horner, in a judgment, directly recommended that the British government carry out an investigation into alleged security failings in the lead-up to the atrocity.
While having no jurisdiction to order the Irish Government to act on the matter, the judge urged authorities there to establish their own probe in light of his findings.
Earlier this month, the Cabinet formally agreed to provide assistance to the inquiry.
Two senior officials from the Department of Foreign Affairs attended the inquiry hearing on Tuesday.
In his opening statement, Paul Greaney KC, counsel to inquiry chairman Alan Turnbull, said it is “highly likely, if not inevitable, that the state authorities within the Republic of Ireland hold evidence and bearing upon the inquiry’s terms of reference”, adding that access to materials and citizens requires Irish co-operation.
He said inquiry staff have held a number of meetings with Irish government officials and the Irish police, and commended “positive discussions”.
“The discussions that have taken place have been positive, and during them, the government of Ireland has expressed a strong desire to co-operate fully with the inquiry,” Mr Greaney said.
“The challenge is to see the engagement that has occurred to date translated to meaningful engagement with the inquiry’s processes, both leading up to and during the course of inquiries or evidence hearings.
“We emphasise that the inquiry team does not doubt that the Government of Ireland has a strong commitment to achieving that vital aim.”
He added: “The intention of the inquiry is to ensure that those discussions now move into a practical phase in which clear written agreements are put in place, to make certain that the inquiry has full access to any potentially relevant materials held by the state authorities in the Republic of Ireland, and that it has the ability to obtain statements and call witnesses to attend the inquiry, to give evidence in person.”
