Dún Aonghasa (also written Dún Aonghusa) is a spectacular Iron Age cliff fort on the western edge of Inis Mór, 8 km from Kilronan by the main road. Three concentric semicircular stone walls end abruptly at the edge of an 87-metre sheer cliff — the ocean is the fourth wall. The innermost enclosure is remarkably well preserved, and the view from the cliff edge, looking straight down into the Atlantic, is one of the more vertiginous experiences available in Ireland. The fort dates to around 1100 BC and was modified through the Iron Age and early medieval period.
The approach from the car park is a 15-minute walk uphill on a well-maintained path. In summer the site is busy, but the crowds thin quickly as you walk through the outer walls toward the cliff edge. There is an entry fee (around €5 for adults). By bike from Kilronan the ride takes about 45 minutes; the road is mostly flat heading west and very slightly uphill on the return.