I cannot find out anything about her. This is an early work of noted portraitist Joseph Karl Stieler.
From Pinterest search. Also posted to the Revolutionary, Napoléonic, and Romantic Era Album here.
This dress has a constructed bodice although it looks like it does not. The lace collar joined to the lining is interesting.
Keywords: 1805, Stieler, Marianna Dembińska, Moszyńska family, Dembińska family, Polish, curly coiffure, tiara, feathered headdress, ribbon headdress, necklace, vee neckline, lace collar, lining, elbow length close sleeves, bracelets, order bows, waist band, Empire low waistline, close skirt, cameo, wrap