This little town, so famous throughout Europe in the eighth and ninth centuries as a school of saints and doctors, is situated on the left bank of the Shannon, about ten miles south of Athlone, almost in the centre of Ireland. It gives its name to the small and ancient See, which, conjoined with Ardagh, now forms the diocese of Ardagh and Clonmacnois in the province of Armagh. Its patron, St. Ciaran, or Kieran, born in 516 (one account gives his birth as 483, and says he was baptised by St. Patrick), died in 549 after having founded in the previous year his celebrated Abbey of Clonmacnois.

Another earlier St. Ciaran, a contemporary and afterwards a companion of St. Patrick, is said to have built a church here ; but his labours; were more generally confined to the southern portion of the country. St. Patrick, on his first journey into Connaught, crossed the Shannon at this point. Here, too, according to tradition, he set up a Cross where had been a pagan monolith, and baptised a great number of the natives. (It is curious to note that learned antiquaries are of opinion that the Celtic Crosses, so numerous in Ireland, superseded in the ninth and tenth centuries other Crosses of wood or rough stone which had been erected by the early missioners wha assembled their neophytes around them before churches had been built.)