Birr Castle is a large castle in the town of Birr in County Offaly, Ireland. It is the home of the 7th Earl of Rosse and his family, and as the castle is generally not open to the public, though the grounds and gardens of the demesne are publicly accessible, and include a science museum and a café, a reflecting telescope which was the largest in the world for decades and a modern radio telescope.

Birr is a town in County Offaly, Ireland. Between 1620 and 1899 it was called Parsonstown, after the Parsons family who were local landowners and hereditary Earls of Rosse.