Inis Cealtra — Holy Island — sits low in the water about two kilometres west of Mountshannon, and it is the single most extraordinary place on Lough Derg. The island was settled by St Caimin in the 7th century and grew into a significant monastic community that survived raids and centuries of change. What remains today is a remarkable cluster of ruins: four medieval churches, a 25-metre round tower, a saint's well, and a hermit's cell, all packed into a few acres of grass and wildflowers.
The largest building, St Caimin's Church, retains carved Romanesque stonework of real quality. The round tower, though roofless, is largely intact. You can walk the whole island in an hour and read the landscape as a kind of open-air museum. Boat trips run from Mountshannon harbour; the crossing takes about ten minutes.