Enda Kenny leaves the office of taoiseach having broken key promises on disclosure to the Kingsmills massacre families, they have said.
Ten Protestant workmen were killed by the IRA as they returned home at Kingsmills in south Armagh in 1976.
Mr Kenny yesterday formally announced his resignation as Irish premier in the Dail.
In March 2015 he met Kingsmills families in Bessbrook and promised full disclosure from Irish security files on the attack.
Many aspects of the atrocity centre on the Republic of Ireland; the gunmen used it as their base, four weapons were recovered there, the getaway minibus was hijacked and dumped there and the top three suspects served time there for terrorism.
Alan Black. who was shot 18 times but survived, was disappointed with Mr Kenny.
“Sure he wasn’t much good to us at all,” he said. “I think they were that much up to their eyes in it that they cannot release more.
“I met him twice and he was on the phone once too. He made the same promise to me three times. But all they have given us is newspaper clippings and very little else.”
Colin Worton, whose brother Kenneth was killed, said: “We will not be sad to see him go. He never followed up on what he promised us. He promised us full disclosure but that never happened. He was not as honourable as he let on to be.”
The taoiseach declined to offer any comment.