A master physician in Celtic mythology, also called Labhra in Charles Squire's Celtic Myth and Legend. His only appearance is in one of the tales of Finn mac Cumhail and the Fianna.
The Fianna were defending the many children of Midir, the god of the underworld, from the other Tuatha Dé Danann. Many, gods and mortals both, were hurt: Conn son of Midir was slain, two of the Fianna were near death, and the rest of Finn's company covered in wounds.
Another of Midir's sons, Donn, suggested taking Luibra from the gods' camp to tend to the Fianna. His brothers Aedh and Flann, with the human Caoilte, kidnapped the healer as he gathered herbs.
Finn and Donn welcomed Luibra, and forced him to heal the two dying Fianna in exchange for his own life. They were completely recovered in nine days, and Luibra lived.
Luibra is mentioned as being young, strong, well-armed, and possessing a cloak woven from the wool of seven sheep from the Land of Promise. Beyond that, nothing is known.
Sources:
Gods and Fighting Men: The Story of the Tuatha De Danaan and of the Fianna of Ireland, by Lady Augusta Gregory (http://www.sacred-texts.com/neu/celt/gafm/)
Celtic Myth and Legend, by Charles Squire