Many of the Pre-Raphaelite paintings were spread thinly around a large number of collectors because of the relatively low prices compared to some of the great European masters. It was only relatively recently that several larger collectors started to snap them up and group them back together. The Tate is one such collector, though much of their collection was actually gifted by British owners upon death rather than purchased privately.
One of the significant aspects of The Magic Circle is that is was Waterhouse's first submission to the Royal Academy having recently become as associate. It was, of course, the first in a long line of successful submissions to this institution who were particularly keen on the style of Pre-Raphaelite art. The combination of elegant female models and charming artistic style was hard to turn down and the UK was gifted with several skilled members of this group around that time.
The Magic Circle itself, to give the piece its title, is being drawn by the sorceress around her slim figure as she chants to the heavens. She is typically elegant and beautiful, consistent throughout Waterhouse's career. A solid looking cauldron responds by channeling smoke into the air, whilst darkness sets across the background to allow the main focal point to take centre stage.
Celtic mythology had taken plenty of attention around the 1880s and was enjoying somewhat of a revival, well beyond just the interest of Waterhouse. Mythology in general has long inspired artists, be it Celtic, Greek, Roman or perhaps something further afield. Elements of fictional tales seem to fit well with art, particularly the more expressive styles. Waterhouse would always produce flattering, delicate depictions in line with the Pre-Raphaelite movement.