I can only echo Janet W.'s recommendation of Prof. Herbert's "Iona, Kells,
and Derry"--Maire Herbert is a brilliant historian, usually at UC Cork,
but currently here at Harvard for the year as a visiting scholar.
She's a fantastic teacher, and a generous scholar. Run to the bookstore, folks!
K. HUghes' book is also not to be missed--very clear writing, great source.