Her article in thepeerage is here. An image of her mother, Lady Elizabeth Montagu, is here. Her mother-in-law, also Lady Mary, is seen here.
From 18thcenturyblog.com/2012/03/george-romney, probably from Philip Mould; dandruff laundered throughout image by gogm with Photoshop.
This is an example of 1780s fashion being brought forward into the 1790s. The long bouffant coiffure adorned with a head band, the long close sleeves, and full skirt would all fit a decade earlier. She appears to be wearing a criss-crossed fichu over a chemise that completely fills in the neckline area and adds a ruff and a silk jabot. Her dress is predominantly tan, but it has a prominent maroon waist band.
Keywords: Romney, Scott family, Stopford family, Countess, British, curly coiffure, head band, high enclosing neckline, chemise, ruff, jabot, fichu, long tight sleeves, waist band, vee waistline, full skirt