The beautiful mountain of Benwisken is a part of the Dartry Mountain range in north Sligo. The peak of Benwisken blocks entrance into the Gleniff valley, a huge glacier-formed hollow in the north side of the mountain range. The rock is Dartry limestone, and the peak of the mountain is sculpted like the crest of a wave.
The peak of Benwisken is 514 meters above sea level. The tip of the mountain looks like the edge of a knife, or perhaps a wizard's hat. The southern slopes within the Gleniff Horseshoe are fairly gentle, and this is the easiest way to climb; but the cliffs to the north are amazingly steep and jagged. There are many ancient sites scattered about the base of the mountain: medieval cashels, stone walls, megaliths, and the church founded at Keelogues by St. Molaise of Innishmurray.
The mountain is covered with a mantle of blanket bog that holds water like a sponge. The top of the mountain is covered in sink holes and swallow holes, where water drained down into the stone, carving out the gallerys and tunnels behind Diarmuid and Grainne's cave.