Then, with a flourish of two ministerial fountain pens, centuries of England's longest conflict and Ireland's protracted convulsions were tentatively consigned to rest in sleepy university libraries.
The business of the day, in the baroque ballroom of Iveagh House, home of Ireland's Department of Foreign Affairs, was the formal enactment of the Good Friday Agreement by Mr Mandelson and the Irish Foreign Minister, David Andrews.
It was a setting overlaid with paradox: the ballroom's Algerian onyx, Carrara marble and extravagant rococo, which cost the Guinness family pounds 30,000 in 1896, turned their town house into one of Ireland's greatest indulgences. Yet the strident antipathy to Irish independence of the first Lord Iveagh, Edward Cecil Guinness MP, led them to abandon it and move to Britain in the 1930s.
Veterans of the long and tortuous march to peace - including the past premiers Garret Fitzgerald and Albert Reynolds, whose efforts often generated cool responses in London - nodded agreement alongsidse foreign ambassadors and Northern representatives of all political shades as Mr Mandelson, the Secretary of State for Northern Ireland, said voters had "overwhelmingly chosen agreement, consent and peace over division, coercion and violence".
Immaculate Irish schoolchildren from the Catholic, Protestant, Jewish and Muslim faiths smiled as breakfast-time champagne appeared. In response to his "Cheers!", they taught a curious member for Hartlepool his first word of Irish - Slinte ("Your health").
Soon after, at a cabinet meeting in the nearby Government Buildings, the Irish Republic's controversial territorial claim to Northern Ireland was signed away into a gentler aspiration for democratically achieved unity, "in harmony and friendship", in both jurisdictions.
Bertie Ahern, the Irish Prime Minister, was careful to remember the dead before heralding a brighter future. Reconciliation, he said, needed "genuine acknowledgement of the hurt suffered on all sides". Dublin was considering an annual day of remembrance for victims of the Troubles, he said.
Insisting on truth as well as reconciliation, Austin Currie, a former SDLP politician and later a Dublin junior minister, who in 1974 narrowly escaped a landmine near his home, asked: "What has violence achieved? The total futility of violence, when we compare Sunningdale [the ill-fated 1973 attempt at power-sharing] to the Good Friday Agreement: was it worth the loss of one life, never mind over 2,000?"Reuse content