The Kingdom of Uí Failghe or Uí Failge /ui ˈfalʲɡe/ was a Gaelic-Irish kingdom which existed to 1550, the name of which (though not the territory) is preserved in the name of County Offaly, Ireland.
County Offaly (originally King's County) was constructed from a number of ancient Irish Kingdoms of which Uí Failghe was one amongst more such as Mide (present day north Offaly) and Munster (present day south Offaly). In modern territorial boundaries the kingdom today would correspond with east County Offaly, western parts of Kildare and parts of north east Laois. The name was also retained in the names of two baronies in County Kildare that formed part of the kingdom, Offaly (Ophaley) East and Offaly (Ophaley) West.
Kingdom of Uí Failghe Wikipedia
Uí Failghe may have existed as a kingdom since the early historic era, and successfully fought off encroachments by the Uí Néill, the Eóganachta, and the Normans. It takes its name from the legendary king Failge Berraide (fl. AD 507–514).
From the mid eleventh century its ruling 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 seat was originally in Rathangan but moved to Daingean with the Norman arrival. On the death of the last de facto king, Brian mac Cathaoir O Conchobhair Failghe about 1556, Ui Failghe was divided between Queen's County and King's County when it was shired by Mary I of England during one of the Plantations of Ireland. A portion of the original kingdom was made part of County Kildare. Upon Irish independence 'King's County' was renamed County Offaly, in commemoration of Uí Failghe.
The old territory of Offaly is described by O'Donovan in his Ordnance Survey letters. O'Donovan notes the territory of Ui Failghe, or Ophaley, comprising the baronies of: Geshill, Upper and Lower Philipstown, Warrenstown, and Collestown all in King's County; Ophaley (or Offaley) in County Kildare; Portnahinch and Tinahinch in Queen's County. O'Donovan cites O'Heerin as giving that Offaly was originally subdivided into seven cantreds: Tuath Geisille (Geashill); Hy-Regan (Tinahinch); Clann-Maoilughra (Upper Philipstown and Portnahinch); Clar Colgain (Lower Philipstown); Tuath-Maighe or Tethmoy (Coolestown and Warrenstown); Magh Aoife, or Fearann Ui Murchain, (northern half of the Ophaley); Tuath-Leighe (southern half of Ophaley).
This gives the kingdom an approximately triangular shape, with a southwest corner in the Slieve Bloom Mountains, a southeast corner at Dún Ailinne, and the River Boyne and its tributaries (Milltown River, Yellow River) marking its northern border.
Keating cites the following family branches as belonging to the O Conchubhar Failghe: O Caomhanaigh, O Tuathalaigh, O Branaigh, Mac Giolla Phadraig, O' Duinn, O Diomasaigh, O Duibhidhir, Muinntear Riain."