SIPTU president Jack O'Connor is to run for chairmanship of the Labour Party, the Irish Independent can reveal.
Mr O'Connor, who is considered a firebrand by many in the party, has not yet received a formal nomination for April's vote - but confirmed he intended to seek one in the coming days and was working on a manifesto.
He will campaign on the idea that "Connolly's party must be the catalyst for a coherent social democratic movement".
Mr O'Connor said that parties on the left of Irish politics needed to learn to "build alliances" in order to achieve their objectives.
By the centenary of Ireland's independence in 2022, the union boss wants full collective bargaining rights for workers, an end to homelessness, a free national health service and one of the best education systems in Europe.
"In order to do it is a requirement for the left to organise itself sufficiently to win the degree of political power necessary to set the agenda," he said.
Asked whether he envisaged the Labour Party doing business with Sinn Féin as part of this left-wing movement, he said: "I don't think it requires everybody who sees themselves as left to join it."
Mr O'Connor said he did not believe Labour would have to re-enter government after the next election to achieve the party's goals, citing Fianna Fáil's confidence and supply arrangement as a way to be "out of office and holding a great lot of power".
Mr O'Connor has clashed with the party leadership in the past and reacting to the news, Labour Party leader Brendan Howlin told the Irish Independent: "Jack and I share the same broad world view, and he will make a very interesting candidate for chair.
"We may not agree on every issue, but as the son of a trade unionist, I believe I could work very constructively with Jack, and that he will bring enormous experience to the role if elected."
Outgoing chair Lorraine Mulligan has already announce she will not seek re-election at the party's annual conference.