She was Jacobite pretender to the UK throne. Her Wikipedia article is here.
Upgrade image posted 21 July 2014 from the lost gallery's photostream on flickr; the temperature was decreased and the exposure increased to make the dress whiter.
The transition from Empire to Victorian is underway as shown by this portrait of Maria Beatrice Vittoria of Savoia with vee waist, full skirt, and bertha looking forwards and puffed sleeves and a single skirt looking back.
Keywords: Maria Beatrice Vittoria, Malatesta, Savoia family, Este family, Stuart family, Countess, Italian, straight coiffure, tiara, feathered lace headdress, scoop neckline, bertha, lace, epaulettes, quarter length puffed sleeves, vee waistline, full skirt, order bows, shallow necklace, gloves, robes