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No, and it's not pronounced the same either: Neifion is 'nay-vyon' (nay to rhyme with may), Fionn is 'fin'. I'm inclined to agree with Rene's theory as to its origin.
Irish and Welsh are both Celtic languages but they're not that closely related, being from different branches, and don't share many common elements. The Welsh equivalent of 'fionn' is 'gwyn'.