The medieval monastery of Clonmacnoise (Cluain Mhic Nóis in Irish, meaning “Meadow of the Sons of Nós”) was founded by St Ciaran between 545 and 548 by the river Shannon and at the crossroads of several medieval routes linking all parts of Ireland. The monastery thrived between the 7th and the 12th centuries. However, it was plundered by the Vikings, the native Irish and Anglo-Normans over the centuries. It finally fell to English in 1552. The site includes the ruins of a cathedral, seven churches dating from the 10th to the 13th centuries, two round towers, three high crosses and what is believed to be the largest collection of Early Christian grave-slabs in Western Europe. The monastery is the burial place of many kings of Tara and Connacht.
There are also the ruins of a Norman castle beside the monastery, however, these are not open to the public. Nevertheless, you can have a good look at them from the jetty on the river Shannon.