Jack O'Connor stepped down as Siptu president on Thursday.
One of the most recognisable faces of the union movement in Ireland, Mr O'Connor served three consecutive six-year terms as a national executive officer of the State's largest trade union, including 14 ½ as general president.
The union said Mr O’Connor was stepping down from his role following a meeting of the union’s national executive council.
From mid-2007 to mid-2011 Mr O'Connor was also vice-president and then president of the Irish Congress of Trade Unions.
“While he is stepping down on completion of his third term, he is not retiring and will continue to work for the union in another capacity,” a Siptu statement said.
Joe Cunningham, who was recently elected general-secretary-designate, will be the union's spokesman on general policy, it added.
Mr O'Connor, who lives in Naas, Co Kildare, is poised to run for the Labour Party at the next general election. He has been asked to put his name forward at a selection convention in Wicklow and says he will attend and do so.
"I want to make it clear I have no intention of muscling in on anyone else," he told The Irish Times in November, adding he would run on a platform of strong Labour Party policies such as housing, health and education if selected.
Mr O’Connor was initially approached by the Wicklow Labour constituency council with a view to standing in the event of a snap general election before Christmas.
Labour sources said it was highly likely that he would be selected. The party held a seat in Wicklow in the last Dáil but lost it in the 2016 general election.