The Queen has offered her sympathies following the death of former Irish taoiseach Garret FitzGerald.
As Prime Minister David Cameron and Taoiseach Enda Kenny paid tribute, the Queen sent a personal message praising Dr FitzGerald's dedication to peace in Ireland.
"I was saddened to hear this morning's news of the death of the Garret FitzGerald, a true statesman. He made a lasting contribution to peace and will be greatly missed," she said.
In a message to Irish President Mary McAleese she added: "Please will you convey my sincere condolences to his family."
The political leaders of Britain and Ireland also expressed their sympathies.
Dr FitzGerald, politician, journalist and economist, who led two governments in the 1980s and fought tirelessly to cement close ties between the two countries, died after a short illness in a Dublin hospital.
Mr Kenny, who had an early-morning meeting with Mr Cameron in the Irish capital, said Dr FitzGerald's only concern was for the people and the country of Ireland.
"Garret FitzGerald was a remarkable man who made a remarkable contribution to Irish life," he said.
"His towering intellect, his enthusiasm for life, his optimism for politics was always balanced by his humility, his warmth, his bringing to public life of a real sense of dignity and integrity, and his interest being focused entirely on his people and on the country.
"He will be judged as being a true republican who was an icon of decency and high honour in public life. The fluency of his economics was always balanced by the humility and generosity and warmth of his personal and family life.
"He had an eternal optimism for what could be achieved in politics. You could not tire him out and his belief that politics and democracy would work for peace."
Mr Kenny also said his former Fine Gael party leader would have been happy to hear the Queen address Ireland last night as part of her state visit.
"To see the work that he had done over very many years, and indeed his father (Desmond) before him, have played their part in putting the jigsaw of peace together."
Mr Cameron, who attended the Queen's speech at a state dinner in Dublin Castle last night, said he watched Dr FitzGerald when he was a student of politics, rather than someone involved in politics.
"He always struck me as someone who was a statesman, as well as a politician, someone who was in politics for all the right reasons and someone who made a huge contribution to the peace process and bringing reconciliation for all that had happened in the past," the Prime Minister said.
"I hope today of all days, with the state visit and the warm relationship between Britain and Ireland that he can see, that some of his work has been completed."
Referred to as "Garret the Good" by colleagues and opponents alike, his death was announced in a short family statement from his children John, Mark and Mary.
"The family of Dr Garret FitzGerald are sad to announce that he has passed away this morning after a short illness," it said.
He had been undergoing treatment in the Mater private hospital over the last few weeks.
Funeral details are to be released at a later date, his family said.
The Irish Tricolour was flying at half-mast over Government Buildings and Leinster House parliament building today.
Dr FitzGerald died two months after his Fine Gael party returned to power with a resounding electoral success.
His name will go down in history for striking the 1985 Anglo-Irish Agreement with former British prime minister Margaret Thatcher, which tied the two countries to closer co-operation on Northern Ireland.
President Mary McAleese said he was the Renaissance man of our time and a national treasure.
"Above all, Garret FitzGerald was a true public servant. Steeped in the history of the State, he constantly strove to make Ireland a better place for all its people," she said.
"Garret was a persuasive voice for progressive reform. As minister for foreign affairs, he anchored Ireland's place at the heart of Europe and enhanced our national reputation in the world, and, as taoiseach, he courageously led the debate for a more tolerant and inclusive Ireland.
"His crowning achievement in politics was his negotiation of the Anglo-Irish Agreement of 1985, which was a key milestone on the road to peace and partnership politics in Northern Ireland."
She added: "I offer my deepest condolences to all of them on their very sad loss and hope they will derive some consolation from the knowledge that Garret lived his life with total integrity, unrelenting purpose and an unquenchable concern for the welfare of Ireland and its citizens."
Politicians on all sides added their sympathies, including Fianna Fail leader Micheal Martin, who said Dr FitzGerald served with great intelligence, decency and commitment.
"Garret FitzGerald has made an enormous contribution to Irish politics and to our wider society," Mr Martin said.
"He was a hard-working politician of compassion and ability. He was a prolific journalist of insight and understanding. He was a brilliant academic of versatility and knowledge.
"He was an implacable opponent of those who espoused violence as a means to a lasting political solution on this island and he worked sincerely towards building an Ireland free of conflict.
"He was a person who cared deeply about Ireland and he has given distinguished and patriotic service to our people."
Sinn Fein president and Louth TD Gerry Adams offered his condolences to family and friends.
Airline Aer Lingus, where Dr FitzGerald began his career and where the legend was born that they took on four men and a computer to replace him, said he would be sadly missed.
A company spokesman said: "Aer Lingus sincerely regrets the passing of our former colleague Dr Garret FitzGerald and would like to extend our deepest sympathy to his family.
"Without any of the modern-day analysis tools, Dr FitzGerald brought his keen economic mind to bear on how best to plan and utilise aircraft, laying the foundations for the future success of the airline in this important area."
Former taoiseach Bertie Ahern described Dr FitzGerald as supportive and helpful particularly during his time in office and in the quest for peace.
"Garret was never partisan or tribal. He truly did put people before politics. Fine Gael were his party, but he recognised that no group or individual had a monopoly of wisdom," Mr Ahern said.
"Garret cared deeply about peace on our island.
"He was generous in his advice and vocal in his encouragement of my efforts to secure what became the Good Friday Agreement and then to get it implemented."
Former UK prime minister Sir John Major said: "It was impossible to know Garret FitzGerald without liking him, and impossible to like him without admiring him.
"In difficult circumstances he sought to end conflict and promote harmony. At the end of his political life he left a legacy of affection and respect for what he was as a man, and what he achieved as a statesman."