Her article in thepeerage is here.
Found on a Google search that did not link to an equal image; image enlarged by one half.
Lady Mary poses in a typically rural-esque British portrait wearing a satin bodice, skirt, and under-skirt for this 1749 Ramsay portrait.
Keywords: 1749, Ramsay, Mary Stewart, Lady Fortrose, Stewart family, Mackenzie family, British, curly coiffure, jeweled headdress, feathered ribboned hat, chemise, square neckline, neckline ruff, long full sleeves, arm bands, cuffs, vest bodice, ruffles, buttons, strap sleeves, full over-skirt, full under-skirt