Her genealogical sketch is probably here.
From artuk.org. Also posted to the Reign of Louis XVI Album here.
This sitter is dressed very casually, some what as sitters were for Lely and Kneller portraits from a century earlier.
Keywords: 1785, Batoni, Cecillia Charlotta Francesca Mahony, Countess Mahony, Princess Giustiniani, Mahony family, Giustiniani family, Countess, Princess, long straight coiffure, earrings, high neckline, collar, bows, full flared upper sleeves, arm bands, long tapered lower sleeves, flared cuffs, Swiss belt, close skirt, fan