Clonmacnoise Monastery
On the banks of the River Shannon in central Ireland, Clonmacnoise was probably the most important monastic site in Ireland.  The site dates back to 548, although most of the remains here date from the 10th-12th centuries.  This site, along with Glendalough, are part of the reason that Ireland became know as the "island of saints and scholars" during the dark ages, as knowledge and art flourished in these places.  There are several stunning monastic crosses here, both of which have been removed and placed in an adjacent museum for protection from the elements- they have been replaced on the grounds with duplicates.  The originals are pictured here.  The monastery survived no fewer than eight Viking attacks, a dozen burings, 27 attacks by native Irish, and it finally died when plundered by the English.