However, that office was abolished in a law change following the Anglo-Irish Treaty of 1921, which in effect aborted Southern Ireland (which had in reality only existed on paper) and established the Irish Free State.

These include a tree (Abies albertiana) planted by the Duke of Abercorn, the first Governor of Northern Ireland, in October 1925.

[11][note 1] Elizabeth II and Philip, Duke of Edinburgh stayed in Hillsborough Castle during their visit to Northern Ireland as part of the Golden Jubilee tour of the United Kingdom in 2002[3] and the President of the United States, George W. Bush, visited the castle in 2003.

[7] The house was also used in January 2010 for talks between British Prime Minister Gordon Brown, Irish Taoiseach Brian Cowen and representatives of the Democratic Unionist Party (DUP) and Sinn Féin on the crisis over Northern Irish policing which threatened to derail power-sharing and to collapse the Northern Ireland Executive.

[13] On 13 September 2022, King Charles III visited the castle with Queen Camilla and responded to an official message of condolence from the Northern Ireland government.