By Laurie McAndish King. Sheela-na-gig’s invitation is fraught with danger. Our relationship began with my quick peek at a wildly pornographic image in Thomas Cahill’s popular book, How the Irish Saved Civilization. An ancient goddess, Sheela is rendered symbolically, stripped of all but the essential features. She is naked, bald, and breastless, and reaches both arms behind her bent legs, using…