Rikhei is looking for how to refer to a female librarian in Welsh over at Ask MetaFilter.
Llyfrgellyddes would be the right word. The male version is llyfrgellydd.
Sorry Paul, but I don’t think there’s such a word as Llyfrgellyddes, or even if there is, it’s never used. Both a male and female librarian would be refered to as Llyfrgellydd