DUP leader Arlene Foster said the “abhorrent nature” of the Kingsmills massacre has not faded over time.
Forty years ago, the Provisional IRA murdered 10 Protestant workmen on their way home from work in south Armagh.
In 2012 in Dublin Mrs Foster and PIRA victims pressed Enda Kenny for a state apology.
“Many victims question the role played by the Irish Republic in the formation of the Provisional IRA, but also the ease with which republican terrorists were able to cross the border to launch attacks, and to retreat southwards to an effective ‘safe haven’,” she said.
Progress will not be made in dealing with the past “if we do not include the Republic of Ireland in that process”.
She noted that nobody was ever charged with Kingsmills but adds “there can be no deadline on justice”.
The MLA said she “will always be a voice for innocent victims” and while many still desire to see justice “we can all share a common goal however, never to see such horrors repeated in our land”.
Victims’ campaigner Willie Frazer said the same gang carried out the Tullyvallen massacre and 24 other murders in the Whitecross, Newtownhamilton area, in only a few months.
He said: “The question that has to be asked is how on earth did these men get away with these crimes – some of whom had never been arrested – even though the dogs on the street knew who they were?”
One Kingsmills widow in her 90s begged him: “At least name them and shame them, for what they did to my husband you wouldn’t do to a dog.”
He said Dublin has “reneged” on promises to share all its Kingsmills files with the pending inquest in Belfast.