Clonmacnoise lies on a bend of the River Shannon about 20 km south of Athlone, and for around 600 years it was one of the most important monastic settlements in Europe. Founded by St Ciarán in 548, it grew into a centre of learning, manuscript production, and metalwork that drew pilgrims and scholars from across Ireland and beyond. Vikings raided it repeatedly; the Anglo-Normans plundered it; Cromwell's army tore apart the cathedral. What remains is still remarkable.
The site spreads along an esker ridge above the Shannon: the cathedral, seven churches in various states of ruin, two round towers, and a collection of high crosses that rank among the finest in Ireland. The original sandstone crosses — Cross of the Scriptures, South Cross, North Cross — are displayed in a purpose-built visitor centre to protect them from further weathering; the ones you see in the open air are accurate replicas. The atmosphere at dawn or dusk, with mist rising from the Shannon and the rooks settling in the ruins, is genuinely otherworldly. The site is managed by the Office of Public Works; admission fee applies.