[Ir., black one of the lake (?); black duck (?)].
Beautiful daughter of Fiachna mac Báetáin and wife of Mongán, born the same night as her husband; celebrated for her white arms. Brandub, who coveted her, deceived Mongán into giving his wife to him. Mongán, however, a son of Manannán mac Lir, sought the help of the hag Cuimne in recovering her. Cuimne transformed herself into a beautiful woman and offered herself to Brandub in exchange; once Dubh Lacha was free, Cuimne reverted to her former shape, a rare instance of such a reversal. This tale is told in Tóruigheacht Duibhe Lacha Láimh-ghile [The Pursuit/ Rescue of Dubh Lacha of the White Arms], ed. Seamus Ó Duilearga, Zeitschrift für celtische Philologie, 17 (1928), 347–70.