Clonmacnoise is a medieval monastery situated on the river Shannon south of Athlone in Ireland. The site contains ruins of churches, numerous Christian graves and beautiful Celtic crosses from the Middle Ages. The monastery Conmacnoise was founded by St. Ciarán in 548 and, due to its privileged location, experienced a rapid growth that lasted until the 12th century despite frequent attacks by the Irish, the Vikings, and the Normans. It was finally destroyed by the English garrison at Athlone in 1552.