Another portrait of her is here. Her article in the peerage is here.
Upgrade image from Wikimedia posted 7 May 2013; image made lighter by gogm.
Lady Isabella wears a full skirt, but without awkward panniers; the stomacher treatment of arcs of ruching is very 1700s.
Keywords: 1769, Gainsborough, Isabella Stanhope, Isabella Molyneux, Viscountess Molyneux, Countess of Sefton, Stanhope family, Molyneux family, Viscountess, British, high straight coiffure, square neckline, modesty piece, revers, ruching, stomacher, elbow length tight sleeves, bows, engageantes, lace, full skirt, over-skirt, maxi-length skirt, shoes, choker necklace, earrings, wrap