Eyebrows were wildly important during the Elizabethan period, which is why Shircore paid special attention to this feature on all the actors. "We obviously bleached out Margot's eyebrows to give her a softer eyebrow to go with her red hair, but we left them quite full at first because that was the young Elizabeth and we were gradually changing her. So once we've got the boils, and the blisters, and the scarring, we then made them up into those eyebrows, her lovely full eyebrows, and started to reduce them by using various waxes to cover bits of hair so it would look like her eyebrows were slightly straggly and thinner. That straggly eyebrow look went well with the dry, brittle, thinning wig because she started to lose her hair after smallpox."
She also paid close attention to the brows of Ronan's character, Mary Stuart. "With Saoirse, we plucked a little bit of her eyebrows, and we bleached her eyebrows as well. She was more pragmatic [with her look], but we did that because a lot of the fashions that developed in that time period were followed by what the kings and queens did."
Turns out, there are some things never change.