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
Mary Stewart in her
Merlin series."
I've never seen an actual Gaelicization of
Jennifer before, but maybe someone else might know if one exists.