Breathnach or Bhreathnach (meaning Welshman) is an Irish surname, indicating an ancestor who was Welsh. 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." (Bell, p. 244). The best known bearer of the name from the area was Uilleam Breatnach (William Wallace).
John de Courcy (1160–1219) planted significant numbers of Britons of Cumbria during his lordship of Uladh. Gaelic-Irish sources such as Dubhaltach MacFhirbhisigh concur, referring to such people as breatnaigh, denoting a Briton (see Old Welsh) (Medieval Ireland, p. 514).