Clonmacnoise was founded by St Ciaran in about 545 AD. He is reputed to have come down the Shannon from his hermit's cell on Hare Island to establish this monastic settlement where the east- west route, the Eiscir Riada, along the esker ridges of central Ireland, met the north south route of the Shannon. Although St Ciaran died seven months later Clonmacnoise grew into a great monastic city famed throughout Europe. 
It was subjected to Viking attacks at the end of the eighth and ninth centuries and again in the tenth century. It was not just the Vikings who attacked; sometimes it was Irish Kings who coveted the wealth of the monastery. 
From the twelfth century it began to decline in importance and, finally, in 1552, the place was sacked and looted by English soldiers from Athlone. By the end of the sixteenth century the churches were in ruins. The site contains a number of churches varying in date from the 10th to the 17th century, a round tower, a castle, three stone crosses and a large collection of early grave-slabs.  

The growth of Celtic Christianity was a product of the missionary zeal of the early Irish monks.  Irish St Columba found the monastery at Iona, in Western Scotland, and as the pagan Anglo-Saxons were destroying the Christianity brought to Britain by the Romans, the Irish Monks were bringing Christianity back to the British Isles by way of the founding of monasteries.

The golden age of the Celtic Irish Church was from the fifth to the ninth centuries.  From this period there are over fifty Irish saints of the Roman Catholic Church, including people like St Brigid, St Aidan, St Kevin, St Brendan and numerous others.