The eBay posting identified this as Maria Pia Borbone, Countess Trapani based on a handwritten note on the back of the card. However, there were a Maria Pia Borbone, Duchess-in-exile of Parma, and Archduchess Maria Isabella, Countess Trapani, alive at the same time. Princess Maria Pia of Bourbon-Two Sicilies bore Roberto di Parma 12 children, of whom six were mentally disabled, according to her Wikipedia article. She died in childbirth in 1882. She is not listed as Countess of Trapani in any place I can find on the Internet. The cropped front of the card does not make the connection, but a handwritten note on the back does. The sitter looks more like Archduchess Maria Isabella.
From eBay; sepia tone removed and image size doubled.