So this is not really my area, but I just finished Lisa Bitel's Landscape with Two Saints: How Genovefa of Paris and Brigit of Kildare Built Christianity in Barbarian Europe. The second half of the book focuses on Ireland, which is Bitel's original expertise.

It's a really neat and current take on Christianization in Ireland, focusing on architecture and sacred landscapes--the interaction of people with the environment. (Environmental studies is big in medieval lit and history right now.) Here's a review, if you'd like to read a bit more about the book.

Bitel is a smooth enough writer, for an academic, and the book isn't weighed down with Theory or academic language. Overall, an enjoyable and fascinating way to look at the topic.