Manannan Mac Lir, meaning Manannan son of Lir (another god). An Irish deity of the sea. I'm sorry I don't know any more than that about this god. I would recommend looking for english translations of the original poems and stories that his legend stems from. If I'm not mistaken, he's king of the water fae. So looking up celtic/irish water fae as well as his father Lir may give you additional results.
What the other poster is referring to. Let me clarify. Manannan has an extra syllable in there than The Craft's Manon.
Manon was created for The Craft. The movie writers decided that to prevent children and teens from copying what they saw in the movie and getting hurt, they would replace the god/goddess's name with a fake name "Manon". In the making of featurette included in the special edition version of the dvd, it is mentioned that after the movie, this "god" began popping up on the internet and the authors of these sites claimed it to have a variety of origins. But it does not exist in history.