This is probably Christian Methuen, wife of Frederick Irby who was the second Baron Boston shortly after their 1775 marriage.
From crests-and-coronets.tumblr.com/post/161694398371/windypoplarsroom-george-romney-lady-boston. Also posted to the Reign of Louis XVI - 1775 to 1789 Album.
The coronet she carries would probably fall off of or crush her magnificent unpowdered coiffure.
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