Birr is a town in the Midlands county of Offaly in the Republic of Ireland. It was once called "Parsonstown". It is situated on the Camcor River and has a castle which is open to tourists all the year round. Birr Castle is the home of the seventh Earl of Rosse.
A main feature on the grounds of the castle is the great telescope of the third Earl of Rosse, an astronomical telescope with a 72-inch reflector which was, until 1917, the largest telescope in the world.
Flowing past Birr, the Camcor River flows into the Little Brosna which, in turn, flows into the River Shannon.