I cannot find anything about this particulat member of the Plummer family.
From artuk.org. Also posted to the Reign of Louis XV - 1715 to 1774 Album here.
Mrs. Plummer wears a lace veil over a stout bodice partially covered by her luxurious robe. The lace edge of her chemise peeks out from her bodice.
Keywords: Hudson - Thomas, Plummer family, British, straight coiffure, lace veil. robe, full flared sleeves, lace-edged chemise, trapezoidal neckline, bodice, belt, jeweled buckle