Some of the battles between churchmen in Ireland are described in The Vikings – A History, by Robert Ferguson.

There were limits to the severity of fighting between Irish tribes. In contrast, those horrible Vikings crossed the defined line and doing so presumably made them barbarians.

For example, in 807 there was a battle between two communities, during which “countless number” of church leaders and leading men of one community were killed. Actual word used in the book is “slaughter”, suggesting more than just the typical losses in a battle.

In 831 a dispute of some sort over relics at a community fair resulted in many deaths.

A fight between two petty kings involved one of them taking sides in the succession of the other petty kingdom which in turn resulted in destruction of one city in 830 and associated capture of a monastery in 836.

Book says they would burn church crops and belongings all the way up to the “door of the church”, but would not burn down the church. When raiding a monastery, Irish leaders were fine with taking the loot and sacred texts, but did not sell off into slavery any of the monks captured at the monastery.

The distinction, apparently, is that Vikings would not only steal all the valuables, but would then also burn down the church. They also captured anyone who couldn’t run away and sold those poor souls into slavery.

The inference is that the pagan Vikings were on the barbarian side of what was considered acceptable fighting.

The Christian Irish kings were on the acceptable side of that dividing line.