I think Fionúir/Fionnuir could be a good transcription (Anglicizations are Fennore, Fennor, and Finora). It means "white ghost." Jennifer is a Cornish form of Gwenhwyfar (Guinevere), which BtN says means "white" + "smooth," which I've also seen in other places; however, in some places I've also seen it translated to "white" + "phantom/ghost" or something like that. The first comment on Guinevere in the database says it's "From the Gaelic meaning "pale phantom", according to Lady MaryStewart in her Merlin series."
I've never seen an actual Gaelicization of Jennifer before, but maybe someone else might know if one exists.