Read: “How the Irish Saved Civilization” by Thomas Cahill
http://en.wikipedia.org/wiki/How_the_Irish_Saved_Civilization
About St. Columba, patron saint of Derry, and an important historical figure in Ireland, and for this subject.
http://en.wikipedia.org/wiki/Saint_Columba
disclosure: a great, great, great uncle of mine was named “Adomnan” the ninth abott of Iona, and the chief source regarding the life of Columba.
Bottom line: Ireland remained literate and safe, whilst the dark ages destroyed much of previous civilization from Rome.
I'm not so sure. There's a 200-300 year period (during the Dark Ages) when not one written word survives in Ireland from that period. Ref: Professor Mike Baillie (Exodus To Arthur)