Elizabeth Wilmot, Countess of Rochester by a follower of Sir Peter Lely
Pepys described her as the "great beauty and fortune of the North." According to her Wikipedia article, she was born Elizabeth Malet and married the Earl of Sandwich. He was a libertine who fathered four children between 1670 and 1676. He also had various affairs on the side.
From Roy Precious Antiques and Fine Art that dates this to ca. 1670; I located image this based on her marriage date.