Her article in thepeerage is here. She was born in 1787, Hoppner died in 1810, and she married in 1811 so this may be an advertising portrait.
Image from Wikimedia; the image had numerous cracked or crinkled paint flaws that were removed with Photoshop so this "fixed" version could be closer to what Hoppner painted than the painting itself is today. Not all of the flaws were removed, especially in the darker parts of the image.
Mary Anne may be sitting on the ground or floor while wearing an Empire dress with the horizontal bodice seam concealed beneath her waist band.
Keywords: Hoppner, Mary Anne Nugent Temple Grenville, Baroness Arundell of Wardour, Nugent Temple Grenville family, Arundell family, Baroness, British, curly coiffure, chemise, modesty piece, sweetheart neckline, quarter length tight under-sleeves, bodice, U-neckline, bows, quarter length close over-sleeves, waist band, Empire low waistline, close skirt