This image is dated based on estimating Lady Mary is in her middle thirties. Her article in thepeerage is here.
Lady Mary looks at the viewer dressed for colder weather with gloves and a fur-trimmed heavy over-garment. Her dress is a typical Napoléonic era dress with high waistline and close skirt.
Keywords: Sharples, Mary Acheson, Lady William Cavendish-Bentinck, Acheson family, Cavendish-Bentinck family, British, curly coiffure, high neckline, neckline ruff, long close sleeves, cuffs, Empire waistline, close skirt, gloves, wrap, fur