The Ó Diomasaigh originated in the territory of Clanmalier, on the borders of what are now counties Laois and Offaly, and remained powerful in the area until the seventeenth century.
In later years, their allegiance was to the English and they were involved with the newcomers in the massacre of the O’Lalors in Laois in 1577, an action which local tradition says was responsible for their later losses.
(PJ Goode – The O'Dempsey Chronicles) The loyalty of the family to the Crown was short-lived, however, and the Williamite wars later in the century destroyed their power and scattered them.Another source states:[5] The O'Dempsey family derive their name from Diummasach, an 11th-century Uí Failge prince of the Clann Máel Ugra, aka Cenél Maoilughra.
The Clann Máel Úgra, in turn, took their name from Máelaugrai, an Uí Failge chieftain who flourished in the middle of the 9th century.
O'Donovan, in his Ordnance Survey letters, places Clann-Maoilughra in the present Barony of Upper Philipstown in the King's County and the Barony of Portnahinch in the Queen's County.According to historian C. Thomas Cairney, the O'Dempseys were one of the chiefly families of the Ui Failghe who in turn were a tribe from the Dumnonii or Laigin who were the third wave of Celts to settle in Ireland during the first century BC.