Clonmacnoise was one of the most important monasteries in early medieval Ireland. Founded by St Ciarán in the mid-6th century, it was located at a strategic crossing over the river Shannon between the provinces of Mide (the midlands) and Connacht along the major land route from east to west. The monastery was subjected to many raids both by Viking fleets attempting to control the Shannon and by Irish kings stealing its treasures or imposing their authority on this important settlement. Kings of Mide and Connacht, many of whom became kings of Ireland, were patrons of the monastery and were buried there. Clonmacnoise also had an active scriptorium in which annals and compilations of vernacular literature were written, among them the 12th-century manuscript Lebor na hUidre (now in the Royal Irish Academy, Dublin).
The remains of the monastic complex, which was at the height of its power from the 9th to the 12th century, consists of nine churches, a round tower, three high crosses, and an extensive collection of cross-slabs, many of which are inscribed and decorated. The main church and Temple Ciarán date to the 10th century and were originally built during the reign of Flann Sinna, king of Ireland and the abbacy of Colmán, both of whom also erected the highly decorated Cross of the Scriptures. The Nuns’ Church, built by Derbfhorgaill, wife of the local king Tigernán Ua Ruairc in 1167, and Temple Finghin, are fine examples of Romanesque architecture. The round tower was completed in 1124 as part of the building programme instituted by Toirdelbach Ua Conchobair, king of Connacht and Ireland, and abbot Gilla Críst Ua Máel Eoin. The monastery’s power declined from the 13th century onwards as it lost out to new religious foundations and its diocese was impoverished.