The death has been announced of Dermot Gallagher, a former secretary general at the Department of Foreign Affairs.
A native of Carrick-on-Shannon in Co Leitrim, Mr Gallagher spent 40 years in government service as a diplomat and in senior positions in government departments.
He is credited with having an influential role in the Northern Ireland peace process.
President Michael D Higgins has led tributes.
In a statement, he said: "Mr Gallagher dedicated a lifetime to public service, serving with distinction as Ambassador to the United States and making a significant contribution to the peace process in Northern Ireland."
Taoiseach Enda Kenny said: "I am deeply sad, as is the Government, to learn that Dermot Gallagher has died.
"Dermot Gallagher was a patriot, an outstanding public servant who embodied the best of Ireland and its people," he said.
Minister for Foreign Affairs Charlie Flanagan said it was perhaps in the area of Northern Ireland and the peace process that Mr Gallagher made his greatest contribution.
"As a young diplomat, he was part of the government's negotiating team at the Sunningdale talks in 1973 and many years later he was a major figure in the negotiation of the Good Friday Agreement," he said.
Tánaiste and Minister for Justice Frances Fitzgerald said that while Mr Gallagher will be remembered primarily for his contribution to peace, he served as chairperson of the Garda Síochána Ombudsman Commission upon retirement as secretary general of the Department of Foreign Affairs.
"He combined a deep respect for An Garda Síochána with a determination that all who served in it should live up to its highest standards," said Ms Fitzgerald.
Fianna Fáil leader Micheál Martin said Mr Gallagher "has left an indelible mark on the diplomatic and public service landscape".
Sinn Féin president Gerry Adams said Mr Gallagher was a "proud Leitrim man who gave a lifetime of service to the Irish State as a civil servant".
Former junior minister and former senator Martin Mansergh, who was also involved in the peace process, said Mr Gallagher had as "emollient style of negotiation and diplomacy".