President Michael D Higgins has met Britain's Queen Elizabeth on the first ever state visit to the UK by the country's head of state.
Mr Higgins and his wife Sabina met the monarch at Windsor Castle three years after the Queen's historic visit to Dublin heralded the start of a new chapter in the relationship between the two countries.
And the significance has been further deepened with the presence of Northern Ireland Deputy First Minister and ex-IRA commander Martin McGuinness, who will attend a banquet hosted by the Queen - a move unthinkable only a decade ago.
The streets of Windsor were decked out in Union flags and Tricolours ahead of the visit.
After travelling to the Berkshire town from London with the Prince of Wales and Duchess of Cornwall, the president and his wife were met by the Queen and the Duke of Edinburgh.
After the guests were honoured with two separate gun salutes, the Queen and President entered the castle in the Australian State Coach, before Mr Higgins and Philip inspected troops of the Queen's Company, Grenadier Guards.
Major Andrew Seddon, captain of the Queen's Company Grenadier Guards, invited Mr Higgins to inspect the guard of honour, speaking to him in Irish.
The Irish Guards was represented by their regimental band as the troops are currently on peacekeeping duties in Cyprus.
Mr Higgins presented a new ceremonial red coat to its regimental mascot, an Irish wolfhound called Domhnall of Shantamon.
Colour Sergeant Frankie Whelan, 36, an Irish Guardsman who was born in Ireland and grew up in County Wicklow before his family emigrated to London 25 years ago, was at the castle.
He said today was a proud day for soldiers from the Republic, adding: "I'm very proud today for all Irish soldiers, whether they be in the Irish Guards or Irish Regiment."
Mr Higgins' extended four-day trip will include an address to the Houses of Parliament and a focus on the contribution Irish emigrants have made to UK life.
The theme of the state visit will be on the two countries' shared histories.
The Queen wore a coat of sky blue cashmere over a moss green and dove grey Paisley print dress, both by Stewart Parvin. Her hat, by Angela Kelly, was decorated with hand-made green feather flowers.