Sorry, it's fiction
Despite the implication in the film, and the real-life local legend, the story is fiction. Historians have determined that the mound at the summit of Garth Mountain (the inspiration for the movie) is a Bronze Age burial mound. In 1999, local officials and the History Society placed a sign on the mountain, telling the many climbers who've been coming there because of the movie's popularity of the site's real significance - and warning that they face two years in prison if they disturb the burial mound. Some maps do identify the 307 meter (1,007 feet) prominence as Garth Hill.
http://www.pentyrch.net/village/garth/map.htm