The Irish Government has vowed to pressure the next British prime minister to agree to let a judge inspect classified security files linked to the worst day of atrocities in the Troubles.
Thirty-three people were killed, including a pregnant woman at full term, when loyalist paramilitaries detonated four no-warning bombs in Dublin and Monaghan on May 17 1974.
The Justice for the Forgotten group has fought a long-running campaign for an open inquiry into allegations that British security agents colluded with the terrorists to plot the co-ordinated and sophisticated attacks.
Ireland's Foreign Affairs Minister Charlie Flanagan laid a wreath at a ceremony organised by the group at the site of one of the bombings on Dublin's Talbot Street.
"I know that the pain of families and of survivors continues to endure. That pain is compounded by the absence - after more than four decades - of the full truth of what happened," he said.
Mr Flanagan said the Irish Government was determined to "continue, and to complete" efforts to uncover the truth about the Dublin-Monaghan bombings.
He said he had pursued the British Government over the last 12 months to give an independent international judge access to all original documents on the atrocities.
"We will continue to do so, proactively and at the highest level, including with the next British government, so that the questions around the attacks will finally be answered," he said.
As well as the Talbot Street bomb, two others were detonated in Dublin on Parnell Street and South Leinster Street. The attacks were co-ordinated in the middle of the evening rush hour.
About an hour and a half later the fourth bomb was set off in Monaghan town. The atrocities were blamed on the Ulster Volunteer Force.
Among those to leave a wreath was U2. The band wrote the song Raised By Wolves about the 1974 bombings.
Denis Bradley, who co-chaired a consultative body on dealing with the legacy of the Troubles, spoke at the commemoration and said issues of "national security" over the opening of police files can be resolved.
"This is not a serious issue to be solved. It can be solved in days or hours," he said.
"People have become very skilled at actually getting around and past that and it is not a serious issue for serious men and women."
But Mr Bradley said he believed the survivors, victims' families and campaigners were a "reasonable distance" from getting to the truth of what happened in 1974.