What is a specific title of a piece of fiction dealing with good versus evil with monastic orders in Ireland?

One title that deals with the monastic orders in Ireland and the battle between good and evil is Byzantium by Stephen R. Lawhead, published in hardcover in 1996 and in paperback in 1997. The protagonist is Aidan, an Irish monk who vies to become one of the monks chosen by the Abbot to take the Book of Kells to the Holy Roman Emperor in Byzantium. Aidan is a pious figure at the start of the book, and he commits himself to an even greater degree of piety to win the honor of transporting the Book of Kells.
Aidan soon meets up with evil, in the form of a Swedish warrior named Gunnar after Aidan's boat is sunk by Vikings. As Aidan approaches the city where Gunnar lives, he crosses himself to protect himself in the barbarian city where he finds himself (140). Aidan watches with disgust as the men in Gunnar's hall, where Aidan is being held as a slave, come to blows with each other. What ensues is a "drunken brawl in a muggy hall reeking of smoke, blood, piss and vomit" (148). It is clear that Aidan is very far from God in these scenes and is witnessing unabated evil.
At one point, a warrior from a group named the Sea Wolves is attempting to drown Aidan in a vat of ale when Aidan prays secretly to God, and Gunnar begins to fight with the Sea Wolf to protect Aidan. While this is ostensibly a fight between two barbarians, it is also a fight between good (the forces protecting Aidan) and evil (in the form of the Sea Wolf and the other barbarians). Eventually, Aidan wins over Gunnar, and Gunnar releases him from slavery. Over the course of the book, Aidan himself wrestles with his faith, while Gunnar becomes a greater believer. Over time, Aidan returns to the land where he was held captive to spread the Christian faith. The book is about the power of faith to turn evil to good.