I have just seen a beautiful 'sheela-na-gig' up here at the top end of 
Australia (I'm currently staying with my son in Darwin).  So the general 
idea seems a pretty widespread one... 

However, what is interesting is that there appears to be two instances 
of what looks suspiciously like a sheela-na-gig - on pictish stones 
(which as per normal are completely ignored in any sort of discussion).  
Unfortunately I dont have my big books with me here so I cant right 
now send on the details.  The dating is probably of the Irish Celtic Xian 
church, possibly 550-700.  And the two pictures appear to be 'angels' 
with wings, squatting on the cross-beam of the cross.  
Of course, it 'could' be a coincidence - but why make an angel squat like 
that?  seems right wierd.  So, would it be off the planet to suggest that 
the sheela-na-gig is some sort of fusion between a pagan goddess and 
the idea of a guardian angel?  (yes yes I know, guardian angels these 
days dont go around doing this sort of thing, but ... the Irish are so 
fond of them, the only really good comfort that catholicism gave us 
when we were growing up :-) 
Helen