Complex sentence: Claire tells Frank of her visit to Craigh na Dun and, Frank's interest piqued by the historical and supposedly mystical powers of the stones, the two decide to visit the site again after seeing Loch Ness.
Split into simpler sentences: Claire tells Frank of her outing, and Frank's interest is piqued by the historical and supposedly mystical powers of the stones. The couple decide to visit the site again after seeing Loch Ness.