Glendalough — the "valley of two lakes" — is one of Ireland's most atmospheric early Christian sites. Tucked into a glacial valley in the Wicklow Mountains south of Dublin, the monastic settlement was founded by St Kevin in the 6th century and flourished for hundreds of years before Viking raids and eventual abandonment left it in romantic ruin.
The round tower, the cathedral, St Kevin's Cross, and the smaller churches are spread along the valley floor between two dark lakes. The upper lake, backed by steep cliffs, is the more dramatic setting — St Kevin's Bed, a cave in the rock face above the water, is where the hermit saint supposedly retreated to pray.
Beyond the monastic ruins, the valley is a superb starting point for walks. The Wicklow Way passes through, and shorter loops around both lakes give you an easy half-day hike through oak woodland and along the lakeshore. Visit early in the morning or in the off-season to avoid the tour bus crowds.