She was a Baroness in her own right and became Marchioness of Buckingham by her marriage to George Nugent-Temple-Grenville, 1st Marquess of Buckingham. She was the daughter of Elizabeth Drax. Her article in thepeerage is here.
From the Fizwilliam Museum; spots removed in background and one spot removed in her cap with Photoshop by gogm.
Lady Mary appears to be wearing a fichu widened to form a cape under a late 1770s-style high and elaborate coiffure adorned with feathers in this 1783 Horace Hone miniature.
Keywords: Hone, Mary Nugent, Baroness Mary Nugent, Marchioness Mary Grenville, Nugent family, Grenville family, Baroness, Marquise, British, long high straight coiffure, feathered cap, vee neckline, neckline ruff, cape, bows, close sleeves