This is the first visit by an Irish head of state to the UK, and follows a 2011 trip in which the Queen became the first monarch to visit the Republic of Ireland.
Wearing an electric blue collarless coat and matching hat, the Queen and President Higgins looked in high spirits as they greeted one another.
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Prince Charles and Camilla, Duchess of Cornwall, were also in attendance after having previously met President Higgins at the Irish Embassy.
Camilla wore a navy blue coat with black piping, which matched with Prince Charles’s suit, as Sabina Higgins wore a pale pink coat with a matching fascinator and cream gloves.
UK and Irish national anthems played as President Higgins arrived at Windsor Palace.
On the eve of his visit President Higgins spoke to the BBC about the importance of his trip, stating that his gift to the Queen would be: "Something equine, something cultural."
He added: "The warmth around this visit has been tremendous."
The Queen smiles widely as she talks to President Higgins
During his stay in the UK President Higgins will address the Houses of Parliament, as is customary for state visits, and will lay a wreath at the tomb of the Unknown Soldier.
He will also meet Prime Minister David Cameron, meet London business leaders and pay tribute to Irish workers.
His planned events have a strong focus on cultural links between the two countries, as he will also attend a celebrate of Irish culture at the Royal Albert Hall and will visit Stratford-On-Avon – the home of William Shakespeare – before returning to Ireland.
Since the Queen’s successful visit to Ireland in 2011, it has been reported that she was keen to be involved personally in President Higgins trip to the UK in order to continue their talk of the close trade and culture links the two countries keep.