The British Ministry of Defence has written to former employees who worked in Northern Ireland in the 1970s, advising them not to respond to requests for information from the campaign group Justice for the Forgotten.
The group is made up of relatives and friends of victims of the Dublin, Monaghan and other bombings, which took place mainly in the 1970s. It has expressed concern over persistent allegations that the bombings were carried out with the collusion of British security forces in the North.
The allegations are currently the subject of an investigation by an independent commission, chaired by Mr Justice Barron, into the atrocities. The commission's report is expected later this year.
Speaking after a memorial garden dedicated to the victims was opened in Dublin's Glasnevin cemetery at the weekend, Mr Greg O'Neill of Justice for the Forgotten said he was disappointed by the Ministry of Defence (MOD) advice. "We had written to former employees of the MOD in Northern Ireland, asking them not for state secrets, but contextual information about that time, and then we received this letter from the MOD telling us they had advised their people not to respond.
"This is particularly disappointing because the British government has not responded to specific requests from the Barron commission."
Mr O'Neill said the lack of co-operation was "an outrage", comparable with the Republic's security forces refusing to co-operate with the inquiry into the Omagh bombing.
Thirty-three people were killed in the bombings in Dublin and Monaghan on May 17th, 1974. One man was killed in a bomb in Dublin on January 20th, 1973 and two others were killed in another Dublin bombing on December 1st, 1972. Two children were also killed in a bomb blast in Belturbet, Co Cavan in 1972; two men were killed in Dundalk in 1975; one man was killed in a blast in Castleblayney in 1976; and one man was stabbed when he disturbed two men planting a bomb on a rail line in Straffan, Co Kildare, in 1975.
Mr O'Neill said the group believed there were links between the bombs in the methods and equipment used, which is why they would like to talk to those in the MOD in the North at that time, to get "contextual" information about methods used by the security forces at that time.
The interdenominational memorial service to dedicate the memorial garden to the victims of the outrages was opened by the Lord Mayor of Dublin, Cllr Dermot Lacey, at the weekend. The mayor said he wished the truth to be established about who planned the murders.