Fianna Fáil Micheál Martin has made a scathing attack on Sinn Féin and the Provisional movement, saying that they have "not the slightest connection" with the Republic declared in 1916.
Mr Martin used the occasion of his speech to his party's annual 1916 commemoration at Arbour Hill today to make a detailed criticism of Sinn Féin and its form of republicanism.
"Unfortunately there have been groups who have claimed allegiance to the Proclamation but have continually undermined its values. They have been deaf to the demand that none who claim to serve the Republic shall dishonour it," he said.
The campaign waged by the IRA was opposed by the overwhelming majority of Irish people, he said.
"The inhumanity of many of their actions, the lasting damage they caused and their sectarian behaviour disqualifies them from claiming to be part of an unbroken chain."
Mr Martin asserted that if people wanted to know where the men and women of 1916 would have stood in later years, they would find out by looking at what they did: taking the route of constitutional republicanism.
"I always find it amusing that another party names cumainn after Constance Markiewicz, but fails to acknowledge that she chaired the founding meeting of Fianna Fáil and was elected as a Fianna Fáil TD," he said.
Mr Martin also criticised Sinn Féin and the DUP for pursuing their own narrow agendas and creating an Executive that he claimed was dysfunctional.
"As a party that claims to promote a Republican ethos, Sinn Féin is also letting down nationalist and republican voters in a very profound way. No number of half-baked Border poll gimmicks should be allowed to distract from this basic fact."
The speech, and its focus on Sinn Féin, was received as an effort by Fianna Fáil to assert its republican credentials against a party which has posed an increased electoral threat in the south.
Afterwards, Mr Martin said it was important that he identified for all members and supporters of Fianna Fáil what the party stands for and how it had evolved as a party, particularly in the context of its republican philosophy, which included Protestant, Catholic and dissenter.
"I will never tire of making the point that the Provisional movement sullied the name of Irish republicanism, undermined it in their capacity to undermine different traditions in Ireland, done damage by their violence, embedded division and bitterness.
"As a result they can never claim to be part of an unbroken chain, which is what Sinn Féin are consistently trying to do in their language and in their rhetoric."