Craig yr Aderyn ("Bird Rock") in North Wales, which is also an Iron Age fort, features in Susan Cooper's "The Grey King". Standing stones on the headland of her fictional Cornish village of Trewissick (apparently based in part on Mevagissey) provide a moonlight shadow "pointer" in "Over Sea, Under Stone":

http://en.wikipedia.org/wiki/File:SusanCooper_OverSeaUnderStone.jpg