[Ir., red stripes].
Ulster Cycle king and foster-brother (sometimes foster-son) of Cúchulainn, best remembered for his tragic marriage to Derbforgaill (1) of Lochlainn. Although she was smitten with Cúchulainn, he could not marry her as he had inadvertently tasted her blood while sucking out a stone that had penetrated her womb. He passed her along to Lugaid Riab nDerg, and the two of them were quite happily married. Derbforgaill suffered a grotesque death, however. Court women goaded her into a test of sexual allure by seeing which woman could send her urine furthest through a pillar of snow; when she won, they jealously mutilated and killed her. On his return Cúchulainn slaughtered 150 of the courtly women, but Lugaid died of grief or shock.
Incest is a frequent motif in Lugaid Riab nDerg's story. He was begotten when his mother Clothra lay with her three brothers, Finn Emna [the three Finns of Emain]. Later Lugaid lay with his mother Clothra to produce Crimthann Nia Náir. This incest may explain his sobriquet. Some texts describe his body as divided by two red lines, separating his head from his shoulders and cutting his trunk at the belt, reflecting the contributions of his three fathers. A prosaic but common alternative is that the red stripes were battle scars.
Modern commentators have argued that Lugaid Riab nDerg was invented to fill out the cast of the Ulster Cycle, aspects of his character being borrowed from other Lugaids. Most specious is his kingship, as he never appears to rule. The Annals record that he was killed by Three Red Heads of Leinster, an apparent contrivance to counter the three Finns [white, fair] who fathered him. See LUGAID MAC CASRUBAE.
See also Carl Marstrander, Ériu, 5 (1911), 201–18.