On the lower western slopes of Knockmoyle mountain in Derreen, County Kerry, one is marked on the Ordnance Survey Fair Plan, that meticulous nineteenth-century mapping effort which recorded field monuments across Ireland with considerable care. The stone is there on paper. On the ground, it is another matter entirely. Surveyors who went looking for it came away empty-handed, defeated not by any great mystery but by something rather more mundane: the area is now heavily forested, and the trees have simply swallowed whatever stands beneath them.