Fianna Fail leader Micheal Martin has branded the people responsible for the murder of journalist Lyra McKee as "cowards" and "cartoon" republicans.
Mr Martin spoke out against the New IRA as members of his party gathered at Arbour Hill cemetery for its annual commemoration of the 1916 Rising.
He told the gathering that "nothing is more disrespectful of 1916" than for blinkered, unrepresentative and fanatical groups to claim any connection to the rising and the national revolution which it began.
He spoke out after the Irish and British governments announced renewed talks aimed at restoring the power-sharing Northern Ireland Executive and Assembly. The talks move came after Ms McKee (29) was murdered by the New IRA as she observed rioting in Derry's Creggan estate on April 18.
Mr Martin said the murder of Ms McKee "is the responsibility of those involved alone".
He added that it is the "sick and inevitable outcome of fundamentalists who no matter how often they wave the Tricolour have nothing but contempt for the Irish people and for... democratic republicanism.
"These people are cowards who hide behind masks and act out a cartoon vision of republican activism. And yet they cause great harm," he said.
He denounced how the New IRA intimidates people who are going about their daily lives, murder "the legitimate forces of law and order" and attack democratic institutions.
"As we saw so tragically on the streets of Derry on the eve of Good Friday 2019, to them a journalist serving the public interest is simply collateral damage. We have to be resolute in fighting these groups," he said.
"We must challenge them at every opportunity and must hold them to account for their behaviour."
Mr Martin said Fianna Fail welcomes the announcement of renewed talks but added: "Clearly Sinn Fein and the DUP have deep problems with how they deal with each other".
Separately, the priest who challenged politicians at Ms McKee's funeral said he needed to hear more from them than complaints about difficulties.
Fr Martin Magill told the BBC he had "pleasant" conversations with both Sinn Fein's Mary Lou McDonald and DUP leader Arlene Foster after the funeral but both spoke of "difficulties" they saw ahead.
He said: "I need to hear more than just difficulties. I need to hear determination. We know there are problems. But we've got to work through those."