I cannot find anything about the sitter.
From the-athenaeum.org; increased saturation, contrast, and temperature.
Lady Elizabeth’s bodice opens on a high-neck under-garment. The bodice neckline may be under a fichu. The wide waist band reaches to the waistline so is this a sheath or isn’t it?
Keywords: 1791, Romney, Elizabeth Haythorne, British, long curly bouffant coiffure, ribbon headdress, chemise, high neckline, elbow length sleeves, cuffs, fichu, vee décolletage, bodice, strap sleeves, epaulettes, waist band, rounded vee waistline, full skirt