According to this article from the (English) National Trust, she was the Earl of Egremont's 17-year mistress and then she became his wife for two years. Then she left, never to return. Her genealogical sketch is here.
From the-athenaeum.org.
Elizabeth is shown wearing an under-dress with half-length mameluke sleeves and a small ruff in this 1797 portrait.
Keywords: 1797, Phillips, Elizabeth Wyngham, Elizabeth Iliffe, Elizabeth Ilive, Countess of Egremont, Countess, British, curly coiffure, ribbon headdress, chemise, neckline rugg, elbow length mameluke sleeves, bodice, strap sleeves, waist band, close skirt