There are many theories surrounding the meaning behind the sheela na gig, and the most popular is that she is a survivor of an ancient pagan Goddess. Usually, the sheela has been identified with the Celtic Goddess Callieach. This Goddess is known to be a "hag" like figure of Irish mythology.
The myth of the sheela says, that she appeared as a lustful hag, and most men refused her advances, except one. After this man slept with her, she turned into a beautiful maiden, and granted the man with royalty and blessed his reign. Before I went to Ireland, I complied a list of sheela na gigs in the places we were going to be visiting. Not knowing how hard or easy it would be to find them, I kept my hopes up. The first place I found one, was at the Hill of Tara.
When you arrive at the Hill of Tara, you have to pass through a small cemetery and old church before reaching the actual mounds of Tara. After we checked out the mounds, we walked back through the cemetery where I noticed a single standing stone. As I looked at it, I realized that there was a slightly faded carving at the bottom right corner. As I looked closer, it revealed itself to me, I had found my first sheela na gig! Very exciting indeed!
The other two sheela's I found on our trip, were at the Rock of Cashel in the museum, and at the Clonmacnoise monastic site. If you set out to find them, you will, just keep looking and dont give up! It's very reassuring that these little figures of an ancient Goddess still adorn the walls of churches and castles throughout the British Isles. For more information on Sheela na Gigs, check out : SheelaNaGig.org
A quote from Celtic Sprite:
The Encyclopedia of Religion, in its article on "ionization" ('womb', 'vagina', 'vulva' or 'belly' in Sanskrit), notes the similarity between the location of many Sheelas on doors and windows and carved female figures wood on the doors of the houses of the chiefs (bai) in the archipelago paluano. Dilukai calls (or dilugai), Open legs typically appear, showing a large pubic area, black, triangular, with hands resting on thighs.
The article's authors say:
These female figures protect the villagers' health and protect them from all evil spirits. The ritual specialists built according to strict rules, which break down resulting in the death of the specialist and the head. It is no coincidence that each instance of symbols representing the female genitalia used as apotropaic resources are located on doors.
The vulva is the main door, the mysterious divide between life and nonlife.
There is some controversy regarding the source of these figures. One view, held by Anthony Weir and James Jerman, is that Sheelas were first carved in France and Spain in the eleventh century, the motif eventually reached Britain and Ireland in the twelfth century.
The work of Weir and Jerman was a continuation of the investigation initiated by Jørgen Andersen, who wrote The Witch on the Wall (1977), the first serious book on Sheela na Gigs.
Eamonn Kelly, Keeper of Irish Antiquities National Museum of Ireland in Dublin, draws attention in his book Sheela-na-Gigs: Origins and Functions of the distribution of Sheelas in Ireland to support Weir and Jerman theory: nearly all Sheelas preserved in situ are found in areas conquered by the Anglo-Norman (XII century), while the areas which remained "native Irish" just a few.
Weir and Jerman also argue in Images of Lust that their location in the churches and ugliness about medieval suggest that standards were used to represent female lust as hideous and sinfully corrupting.