Pronunciation of Samhain is dead tricky. It's one of those that I read in my book on Irish and practise a few times, never sure that I've got it right.

I think it's "sow in", as in the pig family are knocking at the door so let the sow in, but with a nasalized w. Yup, a nasalized w. That's modern Irish. The mutated m was earlier a nasalized v.