I cannot find anything about her, other than this portrait that is. She may be drowned out by another Mary Fairfax.
From the-athenaeum.org; image made lighter by gogm.
Lady Mary's dressmaker made this to show several cm of chemise at the neckline as well as at the sleeves and cuffs.
Keywords: 1741, Mercier, Mary Fairfax, British, straight coiffure, chemise, scoop décolletage, modesty piece, elbow length full under-sleeves, cuffs, bodice, quarter length full over-sleeves, full skirt, wrap