Her parents were actress Dorothea Jordan and the Duke of Clarence who later became King William IV. Her Wikipedia article, including this image is here. She married Charles Richard Fox in 1824.
Lady Mary's dress has the wide shoulders amplified by gigot sleeves and the waist band of a dress of the pre-Victorian era. Her coiffure has the knots in vogue at that time.
Keywords: 1836, Lane, Mary FizClarence, Mary Fox, Welf family, British, hurluberlu coiffure, knotted coiffure, trapezoidal neckline, modesty piece, vee décolletage, collar, lapels, gigot sleeves, cuffs, waist band, Natural waistline, full skirt, bracelets, clasps, handkerchief