From Wikipedia:

Sheela na Gigs (or Sheela-na-Gigs) are figurative carvings of naked women displaying an exaggerated vulva. They are found on churches, castles and other buildings, particularly in Ireland and Britain, sometimes together with male figures. A well-known example can be seen at Kilpeck in Herefordshire, England.

Ireland has the greatest number of known Sheela na Gig carvings: in The Sheela-na-Gigs of Ireland and Britain: The Divine Hag of the Christian Celts – An Illustrated Guide Joanne McMahon and Jack Roberts cite 101 examples in Ireland against 45 in Britain.

Such carvings are said to ward off death and evil (Andersen; Weir and Jerman). Other grotesques such as gargoyles and Hunky Punks are frequently found on churches all over Europe and it is commonly said that they are there to keep evil spirits away (see Apotropaic magic). They are often positioned over doors or windows, presumably to protect these openings.