Gerry Adams is a former chief of staff of the IRA, Fianna Fáil leader Micheal Martin has told the Dail.
Mr Martin made the claim during a debate on the latest allegations of IRA sex abuse.
Mr Adams has consistently denied ever being a member of the IRA despite claims to the contrary by politicians, historians, journalists and authors both North and South of the border.
But using Dail privilege this evening, the Fianna Fáil leader claimed that Mr Adams was in fact IRA "chief of staff".
Earlier, Fianna Fail leader Micheal Martin suggested that Sinn Fein's withdrawal of support for welfare reform in the North is linked to the latest IRA sex abuse scandal.
Mr Martin said Sinn Fein became "spooked" over the prospect that lending support to the welfare changes would damage the party's popularity.
The leader of the Opposition accused Sinn Fein of trying to "essentially tear up" the Stormont House Agreement, adding: "It's an illustration that Sinn Fein does not actually govern".
But in a significant charge, Mr Martin suggested that Sinn Fein's u-turn on welfare reform could be linked to the latest IRA sex abuse scandal that has engulfed the party.
The Fianna Fáil leader said that Sinn Fein has form in creating "distractions" when the party finds itself in trouble.
He gave the examples of the decision to shut down the Dail in the aftermath of the Mairia Cahill affair, and the description of unionists as "b*****ds" by Gerry Adams last November.
Mr Martin described the ordeal suffered by Louth man Paudie McGahon as "harrowing".
Without mentioning Mr McGahon by name, Mr Martin said he had met him and discussed his case.
Mr McGahon has today spoke publicly about his ordeal in an interview with the Irish Independent. His alleged abuse at the hands of an IRA figure - and the subsequent cover-up by Sinn Fein - will be highlighted tonight by the BBC Spotlight programme.
Speaking to reporters today, Micheal Martin said the credibility of Gerry Adams on the issue of abuse is "in tatters".
"I would put it to Gerry Adams that he has a fundamental duty to tell the truth on this. And I would appeal to Gerry Adams not to do on this initially what he tried to do to Mairia Cahill which was to deny, to undermine her in that case," Mr Martin said.
"I was conscious of his closing remarks in his Ard Fheis speech when he said there has been a tsunami of lies and untruths about us and that was directly hit at Mairia Cahill in my view, and there is 'more to come' he said - that was a direct link to Spotlight tonight. I think his credibility is in tatters on this issue as it is on many other issues. I think Sinn Fein has a fundamental problem in telling the truth," he added.
Asked specifically whether the issue of pulling support for welfare reform was linked to the latest IRA abuse story, Mr Martin said Sinn Fein has history in creating distractions from its problems.
"There is a tendency of Sinn Fein, when negative material comes their way, they have a habit of fairly spectacular distractions. We've seen that in recent times. I recall Gerry Adams's comments about unionism using a particular expletive and that was designed to get certain attention," Mr Martin said.
"We had antics in Dáil Éireann on another occasion in the aftermath of the Mairia Cahill affair, if you remember, in terms of 48 hours of distractions and here we are again, one is tempted to say that."