The theme of collusion between the British army and loyalist groups ran through the case of Co Louth man Séamus Ludlow who was murdered in 1976, an Oireachtas sub-committee heard.
Jane Winter of British Irish Rights Watch, an independent non-governmental organisation, said that at the time there had been a toleration of members of the army being members of paramilitary groups.
Many soldiers were part-time in the army and also in paramilitary groups such as the Red Hand Commando. There was very little concern about it and a great deal of toleration.
Nobody was charged with the murder of Mr Ludlow although names of the alleged perpetrators were given by the RUC to gardaí.
She thought Mr Ludlow's murder should receive a public inquiry.
"The failure of the police investigation is not at all acceptable. It should not be for families to have to instigate a proper investigation into a murder," she said.
Ms Winter said a question also had to be asked about cross-Border incursions when Kevin Donegan, Mr Ludlow's brother-in-law, was abducted from Co Louth to Northern Ireland by the UDR and interrogated for an hour.
She said she did not believe there was no record of an interview with Mr Donegan. In answer to a question, she said she thought it would be extremely helpful if an equivalent to the North's Historical Enquiries Team was set up in the State.
She understood the team had already met the Garda Commissioner and were drawing up a protocol for co-operation.
Cormac Ó Dulacháin SC, from Justice for the Forgotten, said the most important issue about the Barron report on the Ludlow murder was that the invitation to interview suspects in Northern Ireland was not taken up.
He said they had identified a number of instances where gardaí went to the North to interview suspects.