• Even though the image is overtly sexual the representation is always grotesque, sometimes even comical. They are usually associated with "hags" or "old women". The carvings often incorporate ribs showing on the torso and sometimes facial scaring as well, although this feature seems to be more common in Ireland than in mainland Britain and the continent.
• The carvings are normally found on Churches usually of Norman origin and of Romanesque design, but they can also be found on Secular buildings (e.g. above a stable door in Haddon Hall Derbyshire, they can also be found on many castles in Ireland )
• Sheela na gigs can be found all over Britain, Ireland and even France and Spain. In fact Images of Lust proposes a Continental origin for them. 
• While the most common interpretation of a sheela na gig is "a pagan idol" there is little evidence for a pagan connection. The prevalence of sheelas in Ireland (far more so than anywhere else) suggests that even if the image originates on the continent the image  has meaning for the Irish. However the pagan interpretation of sheelas is by far the most popular and generates most of the debate on the figures.

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Be sure to check out 

Sheela Na Gig Booklist

Sheela na Gig Theories

Distribution of Sheelas in Great Britain