(CNN) — On forgotten walls of country churches or crumbling castles throughout Ireland, the tiny figures squat unseen.

Lost in gray brickwork, obscured by ivy or moss, Sheela-na-gig stone carvings can be hard to spot in the wild — but these medieval creations are in no way coy.

Typically bald-headed naked females, with hanging breasts and legs spread wide to display exaggerated vulvas, Sheela-na-gigs at first seem peculiarly out of place in the prim surroundings of a Christian church.

However, these envoys from an ancient past have a lot to teach us about Irish and northern European history, and about the pagan roots of the global festival now known as St. Patrick’s Day.

While in modern times it’s a one-day celebration, it was once a three-day carnival that finished on March 18 — Sheelah’s Day.

This is the story of Sheelah — who she was, why she was forgotten when St. Patrick was not, and what traces of her are left behind.

‘She’s always there’

Irish mythology is peopled with…