I've been in and out of the magnificent doorway at St. Mary's for decades and never stopped to look before! There next to the carvings of foul fiends is this immodest lady showing her parts! What is called a sheila na gig from the Irish Gallic
What's it all about, then? Rude ladies represented on church doorways? I imagine sheila na gigs are not primarily a warning against lust etc.. but more likely to be a reminder that "man born of woman has but a short time to live!" All those born of woman will die!
Just inside the door is the original site of the font.
What I think is being communicated is something like:-
When you enter the Church and become one of the baptised
you are no longer subject to the powers of evil and death.
You have been born again - given new life!