Three concentric drystone ramparts raised on Inis Mór more than 2,500 years ago, the open side ending abruptly at an 87-metre cliff falling into the Atlantic. Outside the walls stands a belt of sharp stones driven upright into the ground, a defence against attackers that still works perfectly well on ankles. It is a twenty-minute climb up from Kilmurvey, and there is nothing at all between you and the edge.