The name Galbraith (Rendered in Gaelic, "Mac a' Bhreatnnaich" (son of the Briton)) probably originates from the Britons of the Kingdom of Strathclyde which did not become part of Scotland until 1124.
This description is in accord with the fact that the name is associated from an early date with the ancient kingdom of Strathclyde which had its capital at Dumbarton ('The fortress of the Britons'). It is conjectured that their early chiefs were of the royal house of Strathclyde and by such ancestry they had close affinity with, or married into, the family of the Celtic Earls of Lennox.