Hillsborough Castle is not a castle, but yet another grand Irish Georgian mansion from another age. But it does serve as the royal residence of Northern Ireland. It has the same role as Holyrood Castle in Edinburgh, it's just that few have heard if it. But have a look; its there on the website for Royal Palaces (https://www.hrp.org.uk). And is just one of six open to the public in the UK.
Hillsborough Castle, set at the high point of a quaint period town, is a quick 25 km motorway dash, south of Belfast. On a summer's evening, in this land where flags can say it all, a Lebanese flag is incongruously flying from one of the houses opposite the castle!