Her article in thepeerage is here. She married Thomas Howard, 2nd Earl of Effinghamand then married Field Marshal Sir George Howard.
This is a posthumous portrait.
This portrait dates to well before she died in 1791, based on her coiffure that suggests mid-century, maybe early 1760s.
Keywords: West, Elizabeth Beckford, Countess of Effingham, Beckford family, Howard family, Countess, British, straight coiffure, scoop neckline, robes, elbow length full sleeves, cuffs, over-skirt, mantua, choker necklace, earrings, bracelets, court dress