All I can find out about her comes from Isherwood Fine Arts’ notes, "...Sophia was the daughter of Samuel Dashwood (1725-1793) of Well Hall in Lincolnshire and his wife Anne, daughter of James Bateman who married in London in 1744 and she was one of seven children…"
From 1stdibs.com/art/paintings/portrait-paintings/thomas-hickey-portrait-sophia-dashwood/id-a_1189643/. Also posted to the Reign of Louis XVI - 1775 to 1789 Album.
The bottom of her un-powdered coiffure may be natural, but the rest of it is added on.
Keywords: 1780, Hickey, Sophia Dashwood, Dashwood family, high straight coiffure, ribbon headdress, chemis, modesty piece, U-neckline, bodice, sheer bertha, brooch, draped jeweled bodice ornament, tassels, elbow length flared full sleeves, waist band