Irish President Michael D Higgins paid tribute to IRA victim Lord Louis Mountbatten on the first day of his State visit to the United Kingdom.
While visiting Westminster Abbey, the President and his wife Sabina Higgins paused at a memorial to Prince Philip's uncle, who was blown up on his yacht while on a family holiday in Co Sligo in 1979.
President Higgins bowed his head at the memorial after signing the visitor's book in a highly significant gesture of respect to one of the most prominent victims of republican violence.
After his visit to Westminster Abbey, the President was received at the Palace of Westminster by Speaker John Bercow and his Lords counterpart, Baroness D'Souza.
He then addressed members of the House of Commons and the Lords at a special event in Parliament – the first time Ireland's Head of State has been given the honour – saying the two countries now have a closeness that once seemed unachievable.
The President's speech, watched by Prime Minister David Cameron, Deputy Prime Minister Nick Clegg and Labour leader Ed Miliband, celebrated "the achievement of the momentous Good Friday Agreement of 1998".
"That achievement was founded on the cornerstones of equality, justice and democratic partnership, and was a key milestone on the road to today's warm, deep and enduring Irish-British friendship," he said.
"Our two countries can take immense pride in the progress of the cause of peace in Northern Ireland. There is, of course, still a road to be travelled – the road of a lasting and creative reconciliation – and our two governments have a shared responsibility to encourage and support those who need to complete the journey of making peace permanent and constructive.
"I am conscious that I am in the company here of many distinguished parliamentarians who have made their own individual contributions to the journey we have travelled together," he added.
"I acknowledge them and I salute them, as I acknowledge and salute all those who have selflessly worked to build concord between our peoples.
"I celebrate our warm friendship and I look forward with confidence to a future in which that friendship can grow even more resolute and more productive."
In his wide-ranging speech, President Higgins touched on many of the themes which his four-day official State visit to Britain will explore, including emigration and shared history.
Sinn Fein MPs Pat Doherty, Michelle Gildernew and Paul Maskey were in the audience for the President's speech, along with the DUP MPs Nigel Dodds and Ian Paisley, Alliance's Naomi Long, the UUP's Lady Hermon, the three SDLP representatives Alisdair McDonnell, Mark Durkan, and Margaret Ritchie, and Northern Ireland peers Lord Bew and Lord Eames.
President Higgins talked about his personal connections to the large Irish diaspora living in Britain.
"Generations of Irish emigrants have made their mark on the development of this country," he said.
"As someone whose own siblings made their home here, I am very proud of the large Irish community that is represented in every walk of life in the United Kingdom.
"That community is the living heart in the evolving British-Irish relationship.
"I greatly cherish how the Irish in Britain have preserved and nurtured their culture and heritage while, at the same time, making a distinctive and valued contribution to the development of modern Britain."
The contribution of the Irish in Britain is a recurring theme in the President's State visit to the UK, the first for an Irish head of state in the Republic of Ireland's history.
Today he will visit a central London hospital to meet Irish NHS staff, and after lunch with Mr Cameron in Downing St, will visit City Hall to meet Mayor Boris Johnson and young people from across the UK and Ireland.
Tomorrow he will attend a festival of Irish music, song, dance and poetry in the Albert Hall, and on Friday he will finish his visit with a trip to Coventry to meet the city's Irish community.