A giant Fomori chieftain and grandfather of Lugh; he had one giant, evil eye which drooped, and had to be propped up open, at which point it was used as a weapon, killing all that it looked upon.
Balor's daughter Eithne fell in love with Cian, one of the Tuatha de Dannans, and concieved Lugh. It was prophecied that Lugh would kill Balor, and so the child was sent away. He was raised by Manannan mac Lir, who trained him with a magic spear. When the de Dannans fought the Fomorians the First Battle of Magh Turedh, Lugh came to their aid and used his magic spear to put out Balor's eye, killing him.
This story of course reminds one of that of Perseus the son of Zeus, the Greek hero and ancestor of Hercules/Herakles, who was concieved by Danae in a shower of light and set adrift in the sea after it was prophecied that he would kill his grandfather, who he ends up killing anyway. Both Lugh and Perseus are types of golden children and demigods (Lugh being half-Fomori) of light. Like Perseus, Lugh's offspring is a famous warrior--Cuchulainn.
The counterpart of Lugh would be Bres the Beautiful, who is also half Tuatha de Dannan (his mother is Brigit) and half Formorian. However, Bres represents the dark side, while Lugh represents the light. (see w/u on Lugh for more analogies)
There is also the analogy to the Welsh tale of Culhwch ac Olwen, wherein Culhwch plays both the role of Cian (the lover), Olwen that of Eithne, Goreu ap Custennin that of Lugh (the revenging relative, here a cousin instead of a son), and Yspaddaden Penkawr that of Balor, the forbidding father and giant.