It's very old.
>Mircea Eliade's The Encyclopedia of Religion (1993) draws parallels between the sheela na gig and the ancient Irish myth of the goddess who granted kingship. She would appear as a lustful hag, and most men would refuse her advances, except for one man who accepted. When he had relations with her, she was transformed into a beautiful maiden who would confer royalty onto him and bless his reign.
https://en.wikipedia.org/wiki/Sheela_na_gig