In the Interactive History Time-line there is a page about the life of St. Patrick ( IH06 – C.E. 406-407 Rhine Freezes–St. Patrick Sneezes). This is a bit of Irish history taken from Thomas Cahill’s How the Irish Saved Civilization.
The Irish received literacy in their own way, as something to play with. The only alphabet they’d ever known was prehistoric Ogam, a cumbersome set of lines based on the Roman alphabet, which they incised laboriously into the corners of standing stones to turn them into memorials. These rune-like inscriptions, which continued to appear in the early years of the Christian period, hardly suggested what would happen next, for within a generation the Irish had mastered Latin and even Greek and, as best they could, were picking up some Hebrew.
Cahill, Thomas, How the Irish Saved Civilization, (1995), p. 164.
Cahill goes on to describe how the Irish soon created synthetic languages that would have made J. R. R. Tolkien quite happy.
The other half of the Ogam story is the migration of the Celts with the Roman invasion. The Irish had lived in Wales. Successive invasions of the Latin Britons and the Saxon Angles displaced tribes west and north to their new homelands. While the Irish seldom used the Ogam alphabet in Ireland, it may have been picked up by their Welsh successors in Wales. Thus it may be that the versions of Ogam collected in the Book of Kells were collected from memories of the recent Irish homeland, and the versions collected in New England were brought by the later Welsh Celts who visited America.