As for Sheela-na-gigs ... they're all almost certainly of 12th century origin (maybe 10th if you push it) and almost certainly have nothing to do with 'The Goddess'.
Don't get me wrong. Sheelas are fantastic things. Unfortunately they (and the church) fall outside the remit of The Mod ant due to their recent age. However, here's one or two:
http://www.megalithomania.com/show_site.php?site_id=917
http://www.megalithomania.com/show_site.php?site_id=765
http://www.megalithomania.com/show_site.php?site_id=949
http://www.megalithomania.com/show_site.php?site_id=778
Well four. I must get to some of the others.
Good sources for Sheelas:
"Images of Lust" by Weir and Jerman
"The Witch On The Wall" by Anderson (I think)
&
"Sheela-na-gigs of Ireland and Britain" by Jack Roberts and someone else (I have lent this to someone so don't have it to hand - therefore not 100% on title either)
see also
http://www.beyond-the-pale.co.uk/satan.htm
for a good general discussion on exhibitionist figures on churches.