Duddy is the standard anglicisation of Ó Dubhda in the north of Ireland, particularly in Counties Derry and Donegal. MacLysaght’s Irish Families records Duddy as the usual Ulster rendering of the name.

MacLysaght and other surname writers have noted that the Ulster Duddy families appear to be of different origin from the main Connacht sept — two independent families bearing the same Gaelic name, each descended from an ancestor called Dubhda (“the dark one”). The evidence is principally surname-distribution: the Ulster concentration is continuous and ancient, and is not accounted for by documented migration from Connacht. A full pedigree for the Ulster line has not been published in the form available for the Connacht sept through Mac Firbis.

A smaller Duddy population occurs in County Kerry, where the more usual Munster anglicisation of Ó Dubhda is Doody. In this region Duddy and Doody appear together in Griffith’s Valuation and in the 1901 and 1911 censuses, and the two spellings can represent the same underlying family.

A Derry Duddy and a Kerry Duddy accordingly share a surname but not necessarily a recent common ancestor; the connection runs back through the shared use of the Gaelic name rather than through any documented pedigree.

Griffith’s Valuation and the 1901 and 1911 censuses record the heaviest Duddy concentration in Counties Derry and Donegal, with smaller numbers in Tyrone, Antrim, and — separately — Kerry. Published diaspora figures are limited; emigration from Donegal to Scotland is well-documented in general terms and likely accounts for a significant share of Scottish Duddy lines.

• MacLysaght, Edward. Irish Families: Their Names, Arms and Origins (Irish Academic Press, multiple editions).
• Mac Hale, Conor. The O’Dubhda Family History (1990).
• Griffith’s Valuation, 1848–64. askaboutireland.ie.
• 1901 and 1911 Census of Ireland. National Archives of Ireland.
• irishgenealogy.ie — civil and Catholic parish registers.