She was associated with a later married British ambassador to Naples Sir William Hamilton. Roaring Vesuvio provides a backdrop for the portrait. She is best-known as Admiral Nelson's mistress. Her Wikipedia article is here.
From Piedmont Fossil's photostream on flickr; right side of upper edge fixed and two spots removed from background with Photoshop by gogm.
Lady Emma stares with her big brown eyes while wearing a casual white dress with a cape over-bodice edged with a flounce under a feathered hat.
Keywords: 1781, Romney, Amy Lyon, Emma Hart, Lady Hamilton, Hamilton family, British, mistress, Sir Harry Featherstonhaugh, Charles Francis Greville, Admiral Horatio Nelson, long curly coiffure, feathered hat, strap, bows, high neckline, over-bodice, frills, long tight sleeves, cuffs, waist band, natural waistline, full skirt