Castledermot village was once home to a vast ecclesiastical settlement, but all that remains of St Diarmuid's 9th-century monastery is a 20m round tower topped with a medieval battlement. Nearby are two well-preserved, 10th-century granite high crosses, a 12th-century Romanesque doorway and a medieval Scandinavian 'hogsback' gravestone, the only one in Ireland. Reach the ruins by passing through the rusty gate on all-too-busy Main St (N9), then walking along the tree-lined avenue to St James' church.
At the southern end of town, the ruins of an early-14th-century Franciscan friary can be seen alongside the road.
