Former Taoiseach Albert Reynolds has died at the age of 81.
His family have reported that the Roscommon-born politician passed away shortly before 3am this morning.
Mr Reynolds played a key role in advancing the Northern Ireland peace process, including the 1994 IRA ceasefire.
On its website Fianna Fáil said of Mr Reynolds: "Without a doubt his greatest achievement was in Northern Ireland and Anglo-Irish relations, signing the Downing Street Declaration in 1993.
"It was Reynolds' determination that gave impetus to the peace process and the establishment of an IRA ceasefire in 1994, followed shortly afterwards by a loyalist ceasefire.
“It was Reynolds' determination that gave impetus to the peace process and the establishment of an IRA ceasefire in 1994.
"Albert Reynolds asked the defining question 'who is afraid of peace?'
"His determination brought about what had seemed impossible," the party added.
Northern Ireland Secretary of State Theresa Villiers paid tribute to his work on the peace process.
Ms Villiers said: "I heard with great sadness of the death of Albert Reynolds. He will be long remembered for his courageous and imaginative contributions to the peace process.
"His work, in close partnership with John Major and the British Government, was critical to its success.
"We all owe him deep gratitude for the role he played."
Mr Reynolds was first elected as a TD for Longford when he was 45 in 1977.
Really sorry 2 hear of the death of Albert Reynolds. Solidarity 2 Kathleen & all their family. Albert acted on North when it mattered. RIP.— Gerry Adams (@GerryAdamsSF) August 21, 2014
I liked Albert Reynolds. The Joint Downing Street Declaration was one of the defining moments in British/Irish history.— Simon Fraser (@TheSimonFraser) August 21, 2014
A Fianna Fáil stalwart, he led the party in two coalition governments.
One reign was with the PDs and another was with the Labour Party.
Mr Reynolds made an enduring contribution to the peace initiative in Ireland.
He was also a very successful businessman with an empire that included dancehalls to the pet food industry.
Living in Longford for most of his life, he was considered a deal maker and is credited with securing billions of funding from the EU.
Mr Reynolds battled for years with Alzheimer's disease.
Towards the late stages of his life, he required 24-hour care and could not recognise some members of his family.
He is survived by his wife Kathleen, two sons and five daughters.