President Michael D Higgins has begun his official state visit in Britain.
Michael D Higgins and his wife Sabina met Prince Charles and the Duchess of Cornwall at the Irish Embassy before an official welcome from the Queen and Prince Philip at Windsor Castle.
Crowds gathered in Windsor carrying both Irish tricolours and the Union flag, as President Higgins' arrival was heralded by a traditional town crier in Windsor.
The President and Queen Elizabeth then proceeded in a horse-drawn carriage procession through the streets of Windsor to the castle.
According to the official British Monarchy Twitter account, more than 850 British Army personnel, 275 horses and one Irish wolfhound named Domhnall took part.
President Higgins inspected the guard of honour at Windsor Castle before taking lunch in private with Queen Elizabeth.
Both will attend a formal state banquet later this evening.
His extended four day trip, three years after the Queen visited Ireland, will include an address to the Houses of Parliament and a focus on the contribution Irish emigrants have made to UK life.
On his departure Mr Higgins said the trip – the first ever by Ireland’s head of state – was about remembering the past but also examining the capacity of the present and what can be done in the future to cement relations.
“We are at a very interesting point in history, when we have, following Her Majesty’s visit to Ireland, such good relations between our people,” he said.
“My hope for the visit at the end of it all is that people will in ever more numbers come to share in experiencing the history, the present circumstances and culture, and do so in ever greater numbers.”
The theme of the state visit will be on the two countries’ shared histories.
Taoiseach Enda Kenny said the first ever state visit by an Irish President to the UK is a "very historic" day for the country.
Although the President has travelled to events in London, Manchester, Liverpool and Scotland last year, these were not official visits.
Sinn Féin's Deputy President, Mary Lou McDonald, said the peace process has led to this historic occasion - but there is still much work to do.
Her party opposed the Queen's visit to Ireland in 2011, but supports President Higgins' return trip to Britain.