David Cameron hailed unprecedentedly positive relations with Ireland today as he and Taoiseach Enda Kenny signed a wide-ranging ten-year co-operation deal.
The Prime Minister highlighted agreement on the future direction of the European Union as the pair talked about the "next decade" document at Downing Street.
It also expressed "continuing and unqualified support" for the Good Friday Agreement and other accords surrounding the peace process in Northern Ireland.
But the two leaders remained directly at odds over Mr Cameron's decision not to order a public inquiry into the 1989 murder by loyalist gunmen of Belfast solicitor Pat Finucane.
The Prime Minister has instead appointed lawyer Sir Desmond de Silva to consider evidence in the case - in apparent contradiction of a 2001 agreement to probe crimes involving state collusion.
Mr Finucane's family have been granted a judicial review of the decision
Speaking after talks at Number 10, Mr Kenny said: "I have a difference of opinion here with the Prime Minister" and pledged to continue making the case for a full review.
Mr Cameron, though he said he understood the opposition to his decision from the family and others, defended his stance.
"It's not the nature of the review that matters; what matters is getting to the truth," he said.
"I don't think it's necessary for us to have a lengthy, judge-led, judicial review, with all the problems and costs and expense and time that that takes."
Aside from that issue - and differences of opinion over Irish and English chances in rugby and horse racing fixtures this week - the pair both expressed optimism about current relations.
Mr Cameron said: "We are determined not to roll back on our heels in this moment but actually roll up our sleeves and make the relationship mean even more."
In the past, leaders of the two nations "would have been talking about political processes, prisons, parades, policing", he said.
"But instead of this there is another 'p' which is an entirely positive agenda between Britain and Ireland."
Pointing out that it was very rare for an Irish leader to visit London in the days leading up to St Patrick's Day celebrations, Mr Kenny said: "It covers a very broad spectrum of reality here...recognising that the relationship between Ireland and Britain is now at an unprecedentedly high level of co-operation and interaction in trade and business which is to the benefit of both."
Speaking about St Patrick's Day on Saturday, Mr Cameron joked about London's Tory mayor: "I can't promise that Boris will dye his hair green but you never know, it's an election so he might."
Mr Kenny later joined business leaders at the Irish Embassy for the London launch of the British Irish Chamber of Commerce, established last year to coincide with the Queen's visit to Ireland.
He said it was time for "a new phase in British-Irish relations".
"Our collective challenge is to work together to build business opportunities in Ireland and Britain that will stimulate growth and create employment," he said.
"The Irish government is committed to helping the Chamber in its mission and I and my colleagues look forward to working closely with the British Irish Chamber of Commerce in the future."