The Irish Government has been urged to give “more than face value cooperation” to the Omagh bomb inquiry.
The UK government ordered an independent inquiry into the 1998 outrage in the Co Tyrone town which claimed 29 lives, including unborn twins.
Earlier this month, the Cabinet formally agreed to provide assistance to the inquiry which is to examine alleged security failings that led a High Court judge to conclude the outrage could plausibly have been prevented.
The first public hearing of the inquiry is set to take place in Omagh on Tuesday.
Mr Gallagher said full cooperation from the Irish Government is vital given the belief that the dissident republican bomb was planned, prepared and delivered in the Republic.
He has called for Garda officers, those involved in intelligence and politicians to assist the inquiry.
“We believe that it is hugely important that the Irish government engage because this was a cross-border crime,” he said.
“It was planned, prepared and delivered from the Republic and the bombers spent less than 40 minutes in Northern Ireland, and they were probably back in the Republic before the bomb detonated.
“On that basis, 80 per cent of the evidential opportunities lie within the Republic so it is important they co-operate not just at face value, but we need full co-operation, the same as we expect here in Northern Ireland.
“We expect that Garda officers and people working in intelligence and even politicians will come and assist the inquiry with their work.”
Mr Gallagher said he intends to press to ensure there are no delays or misunderstandings around the level of co-operation needed.
Disappointment has been expressed with the level of co-operation from the Irish government in probes into Troubles atrocities, including the inquest into the Kingsmill shootings in Co Armagh.
“These are fine words, but in substance what do they really mean?” he asked.
“I’m aware in the Kingsmill inquest the families felt that the Irish government did not co-operate in an open and transparent way that they expected.
“We’re aware of that and we feel there needs to be in-depth discussions with the families and with the inquiry team as to how the Irish government is going to manage this process.”
Mr Gallagher added: “I have told the Secretary of State and I have told the Tanaiste that the British and Irish governments are not our enemies, so let’s work together to get the truth.
“There will be embarrassments in the Omagh inquiry for the British government and for the Irish government, but what are those embarrassments compared to what the families have had to suffer for over 25 years.”
