Divine Serpent man only?

[Psitticine]

Dierdriu of the Sorrows would be a really strange choice. She was the most beautiful woman of her day, but had no other superhuman traits than that.

The Fomoriach/Fomorians were deformed sea monsters. They drove away one race of man from Ireland (the Nemedians) who would later return as the Fir Bolg to win their land back. The Tuatha de Danaan also warred against the Fomoriach after they'd displaced the Fir Bolg as rulers of most of Ireland.

(I've got a Fomoriach mod planned, actually.)

The Greenman is a major figure in modern psuedo-pagan worship. You'll see his image around quite a lot if you come into contact with anything even closely related to paganism, Celtic studies, or even the bastardized stuff they try to pass off as Celtic in trendy New Age stores. It would make a rather good Pretender choice, IMO, and would definitely have strong Nature magic.