List_of_kings_of_Uí_Failghe
Uí Failghe seems to have existed as a kingdom in Ireland 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 seat was originally in Rathangan, County Kildare but moved to Daingean with the Norman arrival.
In the 1530s Brian supported the revolt of Silken Thomas; on his defeat he was pardoned and was given the title "Baron of Offaly".[1]
On the death of the last de facto king, and de jure baron, Brian mac Cathaoir O Conchobhair Failghe, in about 1556, Uí Failghe was split between the modern day counties of King's County (Offaly), Queen's County (Laois) and County Kildare by Mary I of England during the Plantations of Ireland. Two Baronies in County Kildare, Offaly East and Offaly West, retained the anglicised name of the Kingdom. Upon Irish independence King's County was renamed as County Offaly.