Her article in thepeerage is here. She acquired the title of Countess from her first marriage to William Matthias Stafford-Howard.
From bbc.co; enlarged by half.
Henrietta's dress has a fur-trimmed stomacher, fur revers, and fur sleeve trimmings and full lace engageantes in this Ramsay portrait from 1759.
Keywords: 1759, Ramsay, Henrietta Cantillion, Henrietta Maxwell, Viscountess Farnham, Countess of Stafford, Cantillion family, Maxwell family, Countess, British, straight coiffure, lace floral headdress, neck band, fur, square neckline, modesty piece, lace, stomacher, revers, three quarter length sleeves, engageantes, vee waistline, over-skirt, muff