When was this portrait painted? It's a British casual dress. So that could be from the 1750s through the 1790s. According to her article in thepeerage, she was the daughter of a Rector who married a Viscount who was raised to Marquess in 1791. Wikipedia gives the date listed here so this could be a portrait commemorating her as a new Marchioness of the Realm.
From 18thcenturyblog.com/2012/03/george-romney.
The new Marchioness of Donegall wears a British casual tunic dress with a little jacket while her robes are draped over a nearby surface in this 1793 Romney portrait.
Keywords: 1793, Romney, Barbara Godfrey, Chichester family, Marquise, British, long bouffant coiffure, head beand, jacket, quarter length sleeves, chemise, scoop neckline, criss-cross bodice, vee neckline, long tight sleeves, buttons, cuffs, waist band, close skirt, shoes