In Irish, the sept’s collective form of the name ends in -a: Clann Uí Dhubhda, “the descendants of Dubhda.” When the name was first committed to English-language records in the 16th and 17th centuries, scribes often preserved this ending. The Fiants of Elizabeth I record variants including O Dowda alongside O Dowde and O Dowdie.

John O’Donovan used the form O’Dowda throughout his 1844 edition of Mac Firbis’s Genealogies of Hy-Fiachrach, the standard 19th-century scholarly treatment of the sept. Mac Hale’s O’Dubhda Family History (1990) likewise uses the O’Dowda form when naming the family collectively.

Over the 19th century, in civil and ecclesiastical records, the final -a was increasingly dropped in favour of O’Dowd. Today O’Dowda is the minority spelling in Ireland. It survives in particular families in the Mayo–Sligo heartland and in some diaspora lines that preserved the fuller form.

Where O’Dowd reads in English as a single man, O’Dowda preserves the grammatical shape of the original Irish — the collective trailing syllable that marks a sept rather than an individual. This is why O’Donovan and the scholarly tradition that follows him tend to use O’Dowda when naming the dynasty as a whole.

• Mac Firbis, Duald. The Genealogies, Tribes, and Customs of Hy-Fiachrach, ed. John O’Donovan (Irish Archaeological Society, 1844). CELT edition.
• Mac Hale, Conor. The O’Dubhda Family History (1990).
• Fiants of the Tudor Sovereigns, printed in the annual Reports of the Deputy Keeper of the Public Records of Ireland (1875–1890).
• Griffith’s Valuation, 1848–64. askaboutireland.ie.
• 1901 and 1911 Census of Ireland. National Archives of Ireland.
• MacLysaght, Edward. Irish Families: Their Names, Arms and Origins (Irish Academic Press, multiple editions).