For anyone else curious about the subject matter, I found this from the National Museums Liverpool, Walker Art Gallery:

Margaret Wilson of Wigtownshire (1667-85) was a Covenanter. She was sentenced to death by drowning because she refused to acknowledge the church hierarchy. Bound to a stake on the shore of the Solway Firth she was engulfed by the oncoming tide. The Covenanters were a group of Scottish Presbyterians who were determined to resist the influence of the Crown and the established Church of England.

Millais’s wife Effie was brought up in Perthshire and may have encouraged his interest in Scottish history. The subject of the Solway Martyr was a popular one, and first appeared as an illustration for the periodical Once a Week, published in 1862.