Clonmacnoise (klŏnmăknoizˈ) [key], village, Co. Offaly, central Republic of Ireland, on the Shannon River. The monastery founded (548) on the site by St. Kieran became the most famous in Ireland. It survived 1,000 years of raids and invasions, until it was destroyed by the English in 1552. Notable ruins include a cathedral (built 904), several churches, two round towers, three sculptured crosses, over 200 inscribed stones, and a castle (built 1214). The ruins comprise a national monument. The annual feast of St. Kieran is held at Clonmacnoise.