County Offaly (Irish: Contae Uíbh Fhailí, derived from "Ua Fáilghe") is a county in Leinster, Ireland, bordered by seven other counties: Galway, Roscommon, Westmeath, Meath, Kildare, Laois, and Tipperary. Originally the Kingdom of Uí Failghe, it was formed in 1556 by Mary I of England during one of the Plantations of Ireland, it was originally named King's County, after King Phillip II of Spain and the county town was Daingean until the eighteenth century.

The southern part of the county is occupied by the Slieve Bloom Mountains, the northwestern part is the floodplain of the River Shannon and the remainder consists of large bogs including the Bog of Allen and Boora Bog.

Birr in the south of the county is best known for its castle and gardens. The Castle ^*, owned by the Parsons family, (the family bears the title: 'Earl of Rosse') is best known for its 19th century telescope. Before the plantation of modern day Offaly by English settlers, the territories it comprises were controlled principally by the O'Connor clan in the North/East of the county and the O'Carrolls clan in the south.