Birr ("plain of water") 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. Birr is a designated Irish Heritage Town with a carefully preserved Georgian heritage. Birr itself has graceful wide streets and elegant buildings. Many of the houses in John's Place and Oxmantown Mall have exquisite fanlight windows of the Georgian period. The town is known for Birr Castle and gardens, home of the Parsons family, and also site of the Leviathan of Parsonstown, the largest telescope in the world for over 70 years, and a large modern radio telescope.

Birr Castle is the oldest inhabited home in the county. In the 16th century the O'Carrolls of Éile had one of their castles here and this was granted to a Sir Laurence Parsons in the course of the Stuart plantation, c. 1620. Sir Laurence Parsons built most of the structure of the present castle. The castle was twice besieged in the 17th century and one of the towers still shows the scars of the artillery of Patrick Sarsfield, 1st Earl of Lucan, who tried unsuccessfully to take it. The castle remains the seat of the Earls of Rosse and is home to the current peer, Brendan, 7th Earl of Rosse, with family members resident in the demesne. As a family home, most of the castle is only open to the public on special occasions, though three rooms can be visited more routinely through the demesne's visitor centre. The castle's demesne, however, is open to tourists every day of the year, and the gardens contain many fine trees and shrubs set in a landscaped park with waterfalls, river and lake, as well as the great reflecting telescope, the Leviathan of Parsonstown, and the modern radio-telescope, I-LOFAR.

In 1960, renowned photographer Antony, 1st Earl of Snowdon, took his new bride, Princess Margaret, Countess of Snowdon, to meet his mother, Anne, who was resident in Birr Castle. Anne, Countess of Rosse, was the wife of the 6th Earl.