Hillsborough Castle is an official government residence in Northern Ireland. It is the official residence of the Secretary of State for Northern Ireland, and the official residence in Northern Ireland of the British monarch and other members of the British royal family when they visit the region, as well as a guest house for prominent international visitors. From 1924 until the post's abolition in 1973, it was the official residence of the governor of Northern Ireland. Since April 2014, it has been managed by Historic Royal Palaces, and is open to the paying members of the public. Early history Hillsborough Castle, which is located in the village of Royal Hillsborough in the north-west of County Down, is not a true castle. It is a Georgian country house built in the 18th century for the Hill family, Marquesses of Downshire, who owned it until 1922, when The 7th Marquess of Downshire sold the mansion and its grounds to the British government. In buying it, the government solved a practical problem. Under the Government of Ireland Act 1920 a new, distinct region of the United Kingdom called Northern Ireland had been created within the traditional province of Ulster, but minus three counties—Cavan, Donegal, and Monaghan—which became part of the Irish Free State. Executive authority had been vested for both Northern Ireland and its sister region, Southern Ireland, in the Lord Lieutenant of Ireland, who was supposed to be one of two all-Ireland features (along with the Council of Ireland) in the new home rule structure. 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. A new office for Northern Ireland alone was created, that of Governor of Northern Ireland. As the Viceregal Lodge in Dublin became unavailable, physically and politically, a new residence was needed. Hillsborough Castle, though outside the largest city of Northern Ireland, Belfast, was deemed a suitable location. After some renovations, the first governor, The 3rd Duke of Abercorn, moved in during 1925. Upon becoming the official residence of the governor, the building was officially renamed Government House. Within the grounds of the castle are a number of trees planted by residents of, and visitors to, the estate. These include a tree () planted by The Duke of Abercorn, the first Governor of