The origins of Muintir Uiginn or O’Higgins have been obscured by time. The major genealogical sources compiled by Dubhaltach Mac Fhirbhisigh (fl.1643) and Cú Choigríche Ó Cléirigh (fl. 1624-1664) seem to suggest that there were two historical branches of O’Higgins both claiming a common origin in their descent from Niall of Tara, the 126th King of Ireland who died about 406 AD.
Ó Cléirigh’s genealogies report that Muintir Uiginn (O'Higgins Clan) were one of several branches of a population group known as the Cenél Fiachach, a branch of the Southern Uí Néill and that they were poets and men of letters as far back as the 6th century. The founder of the Cenél Fiachach was Fiachu Mac Neill a son of Niall of Tara and the 2nd King of Uisneach (fl. 507-514) in Mide which equates to the modern counties of Meath and Westmeath.
According to The Great Book of Irish Genealogies by Mac Fhirbhisigh the O’Higgins belonged to a sub-branch of the Cenél Fiachach known as the Uí Cheallaigh who appear to have been based in or around the territory known as Fir Cheall (anglicized Fercal), which in the late-medieval period was the territory of Uí Mhaoil Mhuaidh (O’Molloy), in and around the barony of Eglish in west County Offaly.
Writing in 1919 Professor Eóin Mac Neill of University College Dublin argued that the O’Higgins of Fir Cheall were local nobles not of the same sept as their rulers, the O’Molloys, and most likely belonged to some old local stock which made no demand for an ancient pedigree.