The sitter would be Mary, née Close. Her article in thepeerage is here.
I added lightness to bring out detail in her coiffure - what came out is she is wearing what appears to be peacock feathers.
This portrait Martin Archer Shee portrait shows how 1820s style appeared on a middle-aged Lady. She wears a white dress and an orange-tan wrap and what appears to be a peacock feather headdress for color.