Gee (pronounced hard like Indian butter, not soft like half a horse) is a modern expression for a vagina used in the south of Ireland. Nobody knows where it originates. There is no Irish word resembling it, not in the dictionaries or indeed in Ó Luineacháin’s wonderful Ó Ghlíomáil go Giniúint. There is a word found in English dialects, gig, with the same meaning, and some people think its origin lies there.
Recently, some people have tried to suggest on line that gee comes from Sheela-na-gig:
Probably from the ancient Irish Síle na Gig, transliterated as sheela-na-gig: the carvings, often found in churches, of naked women grasping giant exaggerated versions of their naughty bits.
This seems to have appeared first in 2014 in an extremely dim and badly-researched article in the Daily Edge, from which the quotation above is taken: (http://www.dailyedge.ie/irish-slang-origins-1468945-May2014/)
It is hard to see where they got this idea from. It is completely implausible. My guess would be that someone logged on to one or two websites on the origins of the Sheela-na-gig (such as http://www.sheelanagig.org/wordpress/ or http://www.irelands-sheelanagigs.org/) and got the wrong end of the stick. The latter, in particular, is insistent on the link between the name of the Sheela-na-gig and the slang word gee. However, it is also quite clear that its author, Gabriel Cannon, is saying that Sheela-na-gig comes from gee and not the other way round.
Incidentally, since I last read anything about the Sheela-na-gig decades ago, Sheela-na-gig-ology has really come on leaps and bounds. There are lots of new theories and new bits of information, including that an 18th century RN ship was called Sheelanagig after ‘an Irish female sprite’ and that Sheelanagig or Shilling a gig was a popular tune in 18th century Ireland. Perhaps I’ll do some research and post on it, as it really is fascinating.