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 & 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).
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."
- Ireland's History in Maps - The Tribes of Laigen Leinster Series
- Hui Failgi relations with the Hui Neill in the century after the loss of the plain of Mide, Alfred P. Smyth, Etudes Celtic, 1975, pp. 501–523.
- Ui Fhailghe, Uibh Fhaili, etc.; The Name of Offaly, Nollaig O Muraile, in Offaly Heritage:Journal of the Offaly Archaeological and Historical Society, pp. 9–11, volume one, 2003