Clonmacnois is the reason why taking a tour is worthwhile. It's too small and obscure to be mentioned in Lonely Planet although it did make it's way onto the map of Ireland in L.P.

[route Day 1 - Dublin: park & Guiness; lunch in the little town of Moate; monastery of Clanmacnois; town of Galway]

Clanmacnois was set up on the banks of the river Shannon by a peace-loving monk and the fraternity of brothers that he gathered.

"Clanmacnoise is a monastery founded in 548 A.D. by Saint Ciaran. The monastery, situated next to the Shannon River, consists of one cathedral and eight churches, mostly in ruins due to numerous Viking attacks. The last High King of Ireland, Rory O'Connor, was buried on the grounds in 1198 A.D." [yay for google!]

Historical details can be read at:
http://www.iol.ie/~mcgibbon/clon/