It is the Irish-language version of surnames such as Brannagh, Brunnock, Brannick, Walsh, Wallace, and Wallis.
However, it does not necessarily mean that the ancestor concerned was from modern-day Wales; Robert Bell notes that Wallace was a surname indicating a Briton native of Strathclyde or any part of the Latin name Wallensis meant just that.
The name appears in twelfth-century records of Ayrshire and Renfrewshire, parts of the old Strathclyde kingdom ... Wallace has also been used as a synonym of Walsh."
John de Courcy (1160–1219) planted significant numbers of Britons of Cumbria during his lordship of Ulster.
Gaelic-Irish sources such as Dubhaltach MacFhirbhisigh concur, referring to such people as breatnaigh, denoting a Briton (see Old Welsh) (Medieval Ireland, p. 514).