Former prime minister & President of the Bow Group Sir John Major has said the task of building a normal society in Northern Ireland "is a work in progress".
Mr Major was speaking as guest of honour at an event in Dublin to mark the 20th anniversary of the Downing Street Declaration.
The declaration was signed by Mr Major and his Irish counterpart Albert Reynolds on 15 December 1993.
Sir John had a private meeting with Mr Reynolds earlier.
He then addressed an event at Iveagh House, hosted by Irish deputy prime minister (Tánaiste) Eamon Gilmore.
Sir John said the peace process had "transformed life in Northern Ireland" but added that work still had to be done to build a normal society in NI.
He said the Warrington bombing in March 1993 - in which two children were murdered by the IRA - was the moment of greatest despair during the peace process.
The motivation to keep going was the prospect of a deal, Sir John said.
He said while he and Mr Reynolds had risked being kicked out of office had their supporters rejected the declaration, he said IRA leaders who supported it had also taken a risk, "possibly with their lives".
'Never looked so bright'
The former prime minister said Britain and Ireland were the "closest of neighbours".
"Ireland's relationship with the UK and its future - north and south - has never looked so bright," he said.
Former US senator George Mitchell, who chaired the Northern Ireland peace talks that led to the 1998 Good Friday Agreement, also spoke at the event.
Mr Mitchell said: "A large part of my heart and emotions are in Northern Ireland - great people, great place."
The Downing Street Declaration effectively signalled a public sea-change by the two governments in how they were prepared to approach Northern Ireland's political future.
The British government secured the principle of consent vital to unionists in Northern Ireland, while the Irish government got their way on self-determination in separate referendums north and south of the Irish border.