Patricia King is to step down as general secretary of the Irish Congress of Trade Unions (Ictu) after more than seven years in the role.
Ms King was previously a vice president of Siptu, where she had worked for more than 25 years.
In 2015 she became the first woman to be appointed general secretary of Ictu, the umbrella body for trade unions.
On Wednesday she announced her decision to step down from the role during an executive council meeting. She is to remain in place until a replacement is selected in the coming weeks.
Ms King, from Co Wicklow, was a lead negotiator in the Croke Park and Haddington Road agreements between the Government and public service unions.
Kevin Callinan, general secretary of Fórsa trade union, said Ms King had been a “fantastic servant to the trade union movement” over many years.
Mr Callinan said during the early weeks of the Covid-19 pandemic the union leader was “instrumental” in pushing for adequate State supports for the public, such as the Pandemic Unemployment Payment and the wage subsidy scheme to keep workers in jobs.
“She played a key role in ensuring people were protected from the worst effects of the pandemic,” he said.
Labour Party TD Ged Nash described Ms King as an “absolute trailblazer” in the labour movement, from her time as a shop steward to her work as a senior union leader.