Found in a Google search and sourced to her article in thepeerage.
Mary Anne wears a dark pearl-trimmed dress, possibly with a ca. 1820 A-line skirt, for this portrait. Her article in thepeerage dates this to 1830.
Keywords: Mary Anne Nugent Temple Grenville, Baroness Arundell of Wardour, Nugent Temple Grenville family, Arundell family, Baroness, British, curly coiffure, hair jewelry, off shoulder bateau neckline, quarter length full over-sleeves, quarter length tight under-sleeves, waist band, Empire low waistline, close skirt, draped bodice ornament, pendant, jeweled neckline, jeweled sleeves, bracelets, wrap, robes