This may be Mary Elizabeth Thornton Ruthven, Lady Ruthven of Freeland, whose article in thepeerage is here.
From pinterest.com/mizliz211/vintage/; sepia tone removed by gogm. This photo is also at the Metropolitan Museum of Art Web site with notes.
The back of a poke bonnet can be seen, She uses a wide collar to cover the opening of her dark lace shawl. The shawl covers a dress made of narrow striped material.
Keywords: 1845, Hill and Adamson, Lady Ruthven, British, bonnet, high neckline, collar, shawl, lace, close sleeves, full skirt