Her article in thepeerage is here. She died in 1794, not long after this portrait was completed. Her birth date is unknown.
From the Lowell Lisbon 2012 auction catalogue.
Lady Elizabeth wears a bouffant coiffure over a wrap that conceals her neckline region. She is probably wearing a zone bodice.
Keywords: 1787, Romney, Elizabeth Morshead, Olympia Elizabeth Morshead, Morshead family, Campbell family, bouffant coiffure, wrap, high vee neckline, zone bodice, long tight sleeves, full skirt