She was the illegitimate daughter of King William IV. Her article in thepeerage is here.
From www.clanhay.org/history/the-house-of-erroll/attachment/eliza-fitzclarence-1822-with-baby-2; image size doubled and sepia tone removed and spots removed from background with Photoshop by gogm.
The Empire influence hangs on with the narrow skirts, now A-line rather than tubular, and the Renaissance style ruff. The dress probably has sleeves puffed that appear to remain puffed further down, perhaps mameluke sleeves.
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