The former Labour Party General Secretary, TD and MEP Brendan Halligan has died at the age of 84.
Mr Halligan died at home surrounded by his family following an illness.
He was an economist before becoming involved in politics in 1967, and served as General Secretary of the Labour Party until 1980.
He was TD for Dublin South West from 1976-1977, and an MEP from 1983 to 1984.
In later years, he served as the Chair of Bord na Móna and the Sustainable Energy Authority of Ireland.
President Michael D Higgins has paid tribute to Mr Halligan, saying the news of his death "will have been received with great sadness by all those with an interest in politics, economics and social justice".
Taoiseach Micheál Martin said Mr Halligan was "a man who gave his life to politics and the public service with a deep commitment to the institutions of the State".
Former taoiseach John Bruton also paid tribute to Mr Halligan, saying he played a key role in the revival of the Labour Party and described him as an "indomitable political organiser"
The former Labour leader Ruairi Quinn said he had made a formidable contribution to Irish public life and that he would be remembered above all for his tireless efforts to deepen Ireland's engagement with the European Union.
"Gifted with a fine intellect and gregarious personality, he demonstrated an inexhaustible capacity for innovative thinking and leadership in his roles".
Current Labour leader Alan Kelly said Mr Halligan was "a fascinating man with a formidable intellect" and a leading light in the party.
Mr Halligan founded the Institute of International and European Affairs in 1990 and today its Director General Michael Collins said Mr Halligan devoted his life and energy to his work as a politician and thinker.
They said the IIEA would be his enduring legacy and that he took great pride in how it had developed.