Wikipedia
O'Dowd , is an Irish Gaelic clan based most prominently in what is today County Mayo and County Sligo. The clan name originated in the 9th century as a derivative of its founder Dubda mac Connmhach. They descend in the paternal line from the Connachta's Uí Fiachrach. The immediate progenitors of the O'Dowd were Kings of Connacht during the 7th and 8th centuries in the form of Dúnchad Muirisci, Indrechtach mac Dúnchado, Ailill Medraige mac Indrechtaig and Donn Cothaid mac Cathail, before losing ground to their rivals the Uí Briúin.
Genealogically, they are closely related to the O'Shaughnessy and MacFirbis clans. Indeed, the O'Dowd were the main patrons of the MacFirbis clan who produced key works of Irish history such as the Great Book of Lecan and the Leabhar na nGenealach. From the 8th to the 15th centuries, the O'Dowd were Kings of Ui Fiachrach Muaidhe, a sub-kingdom within the Kingdom of Connacht. After their realm was incorporated into the Kingdom of Ireland, they were Lord of Tireragh.