Originally the Kingdom of Uí Failghe, Offaly was shired in 1556 by Mary I of England during one of the Plantations of Ireland, it was originally named King's County, after King Philip II of Spain and the county town was Daingean until the nineteenth century.

Uí Failghe seems to have existed as a kingdom since at least the early historic era and successfully fought off encroachments by the Uí Néill, the Eóganachta, and most especially the Normans. From the mid eleventh century, its dynasty adopted the surname Ua Conchobhair Failghe, or O Connor Faly (they were unrelated to the other notable Ua Conchobhair dynasties of Connacht and Kerry). Their capital was Daingean. On the death of the last de facto king, Brian mac Cathaoir O Conchobhair Failghe circa 1556, Ui Failghe was shired into King's County by Mary I of England during one of the Plantations of Ireland. Upon Irish independence it was renamed as County Offaly.

County Offaly covers an area of 771 square miles of mountains, ancient boglands and waterways in the Midlands of Ireland. The county's Clara bog is one of the largest remaining intact boglands in Western Europe and is recognised as being of international importance. The bogland and wetlands of Offaly are home to many species of birds and wildlife. Born Na Mona (Irish Peat Board) operate a guided tour through the Blackwater Bog on the Clonmacnoise and West Offaly Railway, which gives a fascinating insight into this extraordinary landscape.

The early Christian site at Clonmacnoise was founded by St Ciaran in 545 and is one of Ireland's most important monastic settlements. The site today comprises two round towers, a group of churches and three High Crosses. The last High King of Ireland, Rory O'Connor is buried here. Other monuments and historic buildings of note in County Offaly are the Martello Tower in Bangahar, the Napoleonic Fortifications at Lusmagh, Shannonbridge and Cloghan Castle and the splendid Birr Castle and giant telescope. The Georgian Heritage town of Birr is one of Ireland's most charming inland towns.

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. Birr Castle is owned by the Parsons family, (the family bears the title: 'Earl of Rosse') and 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 Ó Conchubhair Fáilghe clan in the North/East of the county and the Ó Céarbhaill of Ely in the south.

The short story writer, novellist, and playwright William Trevor (KBE) was born in County Offaly, and raised in County Cork, Irish Free State, and has written of his experiences growing up as a Protestant in that time and place.