A former senior diplomat, Mr Seán Donlon, told the Oireachtas sub-committee that be believed there was British security force collusion in the 1974 bombings.
"The British army has always had units and elements in it that engaged in dirty operations."
Mr Donlon said that people at a senior level in the British justice system during the 1970s were aware of collusion going on, and yet did nothing to stop it. He often met a "solicitor in Dungannon" who could bring him for a meal or a drink and point out RUC men talking to other men who "weren't there for the drink".
Mr Donlon said that if the British security forces, even rogue elements, had colluded with those who planted the 1974 bombs, the Republic's sovereignty would have been breached by the British.
"It's tantamount to declaring war. Governments should be able to control rogue elements. For the sake of our national interest this should be investigated."
Mr Donlon worked on the Northern Ireland desk at the Department of Foreign Affairs from 1971 to 1978. He told members of the justice sub-committee he was appearing before them after responding to a newspaper advertisement seeking any relevant parties to come forward.
Mr Donlon worked for the Department for 28 years until 1987. From 1974 to 1978 he was in charge of the Northern Ireland desk, a role which involved collating information on security issues in Northern Ireland, particularly on collusion. Despite this, Mr Justice Barron never invited him for an interview.
While getting the truth about the bombings would prove difficult during any public inquiry, he said, it would not be impossible. On the possibility that an Anglo-Irish cross-jurisdictional inquiry could be set up, he said: "I would not give up hope."
The Taoiseach and British Prime Minister enjoyed a very good relationship. "The nature of Anglo-Irish relations would withstand Ireland taking the rough and tough path on this. Unless it's tried we will never know".
Mr Donlon said inquiries into matters of public concern often worked best when headed by an international neutral figure. If the British and Irish governments could agree terms for a cross-jurisdictional inquiry into the events of 1974, and if it were headed by a neutral figure, it might be very effective.
The chair of the inquiry might be given access to British files denied to Mr Justice Barron, he said. Conclusions could be arrived at based on information in the files, without their content being put in the public domain.
He rejected as "misplaced" Mr Justice Barron's criticism that the Irish government should have raised the 1974 bombings, and the collusion issue, with its British counterparts. Some 833 people were killed in terrorist attacks in the Republic, UK and Northern Ireland between 1973 and 1975 - yet not one of the individual deaths, however tragic, was discussed at inter-governmental level.
He had believed the Barron report would focus on collusion and that the actions or inaction of the gardaí and government of the day would prove secondary. But the opposite was the case. However, part of the reason Mr Justice Barron had not reached stronger conclusions on collusion, and other issues, was because he had been denied access to 68,000 files held by the British.
