Dún Aonghasa is one of the most dramatically positioned prehistoric monuments in Europe: a massive semi-circular stone fort perched on the edge of a 100-metre cliff above the Atlantic, dating from the late Bronze Age (roughly 1100–800 BC). Three concentric walls of dry-stone construction, each several metres thick, step up the hill toward the cliff edge, and a band of chevaux-de-frise — jagged limestone slabs set at angles in the ground — forms an outer defensive barrier.
Walking through the walls and then crawling to the cliff edge to peer straight down at the ocean 100 metres below is genuinely vertiginous. There are no guardrails at the cliff itself — this is Ireland, and you're expected to be sensible. The view is extraordinary: the Atlantic all the way to the horizon, seabirds riding the updrafts from the cliff face, and the karst limestone plateau stretching back toward the interior. Allow an hour for the fifteen-minute uphill walk from the car park and the site itself. There is an admission fee and a visitor centre at the base.