I got this wrong! She was listed as Sarah Elizabeth Talbot, but the original color portrait is available from bbc.co (next image) as Maria Theresa, Countess of Shrewsbury. A brief genealogical sketch of her is here. She was a Talbot who married a Talbot. The name Maria Theresa is popular for Catholic girls and her biographical sketch shows her family was Irish and Catholic.
Lady Sarah's dress is trimmed with a cherusque, a Napoléonic holdover, and has lace-trimmed puffed sleeves and a full skirt.
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