Lugh was the son of the Dagda (son of Iarbonel and Danu/Gaea) and Ethniu (daughter of Balor, the Fomorian sun god), and the younger brother of Bodb Derg (born to Morrigan). Lugh was himself the Celtic sun god.
Battling the Fomorians
During the Second Battle of Mag Tuired (when former Tuatha de Danaan leader and half-Fomorian Bres convinced the Fomorians to attack his former subjects), Lugh grievously injured his grandfather and the Fomorian army general, the giant Balor, leading to a Tuatha de Danaan victory.
Lugh was stated to be the brother of Bodb Derg, who is son to the Dagda and Morrigan. However, Lugh's son Cúchulain is stated to have the Dagda and Ethniu as his paternal grandparents, with Iarbonel and Balor as paternal grandparents. Lugh is consequently half-brother to Bodb Derg.