The presence of both Primes Ministers is a reminder of a remarkable personal relationship between two men who played a key role in the push for peace, and in the process transformed the once frosty relations between Britain and Ireland. Mr Blair is credited with ending British condescension to its smaller neighbour while, under Mr Ahern's governance, Ireland has grown in self-confidence as an equal member of the European Union with a dynamic economy. In 1998, Mr Blair became the first UK Prime Minister to address the Ireland's Parliament. Next week, the honour is reciprocated when Mr Ahern speaks to a joint session of the Commons and Lords.
While Mr Ahern is hoping to win a third term for Fianna Fail in the Irish general election on 24 May, Blair, who plans to stand down as Prime Minister soon, sees the latest power-sharing deal between the Protestant majority and Catholic minority as one of the main achievements of his 10 years in power after previous deals proved short-lived. His popularity has been undermined by the Iraq conflict but also by political scandals, in particular his role in the use of peerages for Labour party donors. So Blair has been eager to bow out with success in Northern Ireland and an end to the conflict in which 3,600 people were killed.
Today's ceremony was made possible when Northern Ireland's main Protestant and Catholic groups, Paisley's Democratic Unionist Party (DUP) and Sinn Fein, the IRA's political ally, reached agreement on 26 March after years of deadlock.
Paisley, who will be Northern Ireland's first minister, and McGuinness, who will be his deputy, are unlikely partners. The 80-year-old Paisley has been an outspoken defender of Northern Ireland's British links. McGuinness, a former member of the Irish Republican Army (IRA), and the mainly Catholic Sinn Fein, want to see the province united with the Irish Republic to the south.
Paisley, until recently, refused to talk to Sinn Fein, which he viewed as indistinguishable from the IRA that waged a bloody 30-year campaign against British rule and was responsible for nearly half of the killings during the sectarian conflict.
The home-rule Assembly was first set up under the 1998 Good Friday peace deal, which largely stemmed 30 years of bloodshed. Britain suspended the Assembly and resumed direct rule from London in 2002, after Sinn Fein offices at Stormont were raided by police investigating an alleged IRA spy ring.
The power-sharing government will run Northern Ireland's day-to-day affairs. The parties have agreed to share out ministries with the DUP running the finance, economy, environment and culture portfolios, while Sinn Fein takes education, regional development and agriculture.