The earliest records of the family place them in the region of Tethba in what is now Westmeath, their lands were in Moyashel & Magheradernon barony, Westmeath. The ancestral clan was called Corca Adaimh ('Race of Adam') and they claimed descent from a son of Niall of the Nine Hostages (High King of Ireland circa 400AD) via Máel Dúin mac Máele Fithrich of the Cenél maic Ercae, who was king of Ailech in Ulster. Máel Dúin's sons included the high king Fergal mac Máele Dúin and Adamh, the Ó Dálaigh ancestor. However, one source claims that Adamh was a son of, confusingly, another Máel Dúin the son of Fergal mac Máele Dúin. The great-grandson of Adamh was called Dálach, from whom the later surname derived. The Ó Dálaigh claimed kinship with the O'Neills and O'Donnells.