Happy Sheelah's Day. This feast, largely no longer celebrated in Ireland, commemorates either St Patrick's wife or mother, who probably didn't exist. So let's celebrate a similarly-named mysterious figure today instead: the sheela na gig!
Other proposed explanations have been that the power of the vulva is a protective force. Or a wildly different interpretation that the proud hag serves as a warning against lust.
Basically, nobody really knows why these carvings were so popular.