The hero of a Scottish Gaelic folktale; he is probably identical with Gawain.
Arthur, who is called the King of Ireland, married a mysterious woman who was brought to him on a bier. He fought and was defeated by a man whom he took to be her lover. Ullabh killed the man who, it transpired, was the queen's brother and the son of the King of Ineen.
The latter later imprisoned Ullabh, but the queen's younger sister freed him and he eventually married her and succeeded Arthur.