President Michael Higgins became the first Irish head of state on Tuesday to make a state visit to Britain, crowning a big improvement in historically fraught relations between Dublin and its former colonial master.
Clashes over British-ruled Northern Ireland saw more than 3,600 killed from the 1960s onward until a 1998 peace deal largely ended the conflict between Catholic groups wanting the province to become part of the Irish republic and Protestant groups determined to keep it within the United Kingdom.
Addressing both houses of Britain's parliament on Tuesday, a privilege only accorded to a few foreign leaders including Nelson Mandela and Barack Obama, Higgins described that 1998 agreement as a key milestone but said there was more to be done.
"Our two countries can take immense pride in the progress of the cause of peace in Northern Ireland. But of course there is still a road to be traveled - the road of a lasting and creative reconciliation," Higgins said during his speech to lawmakers in parliament's ornate Royal Gallery.
"Our two governments have a shared responsibility to encourage and support those who need to complete the journey of making peace permanent and constructive, enduring."
Despite being at peace, Northern Ireland remains deeply divided and still sees sporadic outbreaks of violence.
On Monday, a former British government minister said an amnesty should be offered to all those involved in the 30 years of violence in Northern Ireland in order to help the province move on from its past.
Higgins, who said ties between Ireland and Britain were now "strong and resolute", is expected to discuss Northern Ireland during a meeting with British Prime Minister David Cameron on Wednesday.
His trip follows a historic visit by Britain's Queen Elizabeth to the Irish republic in 2011, the first by a British monarch since Dublin won its independence from London in 1921.
In a sign of how far relations have progressed, former Irish Republican Army (IRA) guerrilla chief Martin McGuinness will join Higgins at many events, including a banquet to be hosted by the queen at Windsor Castle outside London on Tuesday.
McGuinness, now deputy first minister of Northern Ireland, rejected invitations to attend events during the queen's 2011 visit but shook her hand in Belfast in 2012 in a milestone which helped draw a line under the conflict.
Earlier on Tuesday, 72-year-old Higgins, whose office is largely ceremonial, arrived for lunch at Windsor Castle with the 87-year-old queen in a horse-drawn carriage as a guard of honor played the British and Irish national anthems.
His four-day visit will include events to highlight the deep economic, political and cultural ties between the two countries. His trip will also take in Oxford, Shakespeare's birthplace of Stratford-on-Avon and the city of Coventry.