Irish president Higgins arrived in London yesterday for the first ever state visit by an Irish head of state.
He visited Westminster Abbey and the Houses of Parliament, and later attended a banquet in his honour at Windsor Castle together with Northern Ireland’s deputy first minister and former IRA commander Martin McGuiness.
The idea of lighting up the White Tower at the centre of the site came from the Irish tourist board, which contacted Historic Royal Palaces, the independent charity which looks after the Tower of London.
Tower of London media and PR manager Cat Steventon said the tower is not often lit up despite frequent requests, but on this occasion it was felt to be "a good fit". The stunt did not need royal clearance.
A photo of the green Tower was tweeted and Steventon said it has been one of the most popular retweets using the hashtag #IrishStateVisit.
Higgins is also due to meet prime minster David Cameron, business leaders and London mayor Boris Johnson before his visit ends on Friday.