uí

Uí is a Gaelic prefix meaning “descendants of” or “kin of,” used in medieval and early modern Ireland and Scotland to denote a kin group or dynasty. It appears in dynastic names such as Uí Néill and Uí Briúin, signaling a common ancestor rather than a geographic tribe.

Etymology and form: The term derives from Old Irish ua (descendant) and is written Uí with the síneadh fada on the i. In names, it functions as a genealogical marker rather than a standalone surname, indicating membership in a particular kin group.

Function and historical role: The Uí prefix was used to designate expansive kin networks that often held territory and supplied rulers within larger political structures, from regional kingdoms to overarching royal lines. These kin groups could be powerful and influential across different regions, and their prominence is reflected in medieval genealogies and annals.

Examples: The best-known example is Uí Néill, the descendants of Niall of the Nine Hostages, who held considerable influence in northern and central Ireland. Another major kin group was Uí Briúin, ancestors of several royal lines in Connacht. Numerous other Uí-based lineages existed across Ireland, each tied to a common forebear and to specific territory or political aims.

Modern usage: In contemporary Irish and scholarly contexts, Uí appears mainly in historical names and place-names. Many kin-based lineages are now known primarily through the anglicized Ó or O’ forms in everyday usage, but the prefix remains a key indicator of medieval kinship and aristocratic organization in Irish history.