Life was violent in the middle ages. The Vikings were not the only vicious and brutal people around.
In Vikings: Raiders, Traders, and Masters of the Sea, author Rodney Castleden provides a tally of raids by raider. Text also mentions two sites long considered to be the location of a Viking massacre. Further study has led to reassessments of the sites in recent years which provide more context.
Who conducted raids in Ireland around 800 AD?
Book says the Irish Annals described 113 monastery raids in the 25 years from 795 A.D. through 820 A.D. Of that tally about a quarter of the plundering was at the hand of Vikings.
Only a quarter.
The specific tally is 26 raids by Vikings. The other 87 were carried out by various Irish kings. Author says that some of the raids were carried out by monks from other monasteries. Assuming that last point is valid, that means there are at least some monks who were skilled at swinging a sword and parrying a thrust.
Reassessment of Viking massacre sites
At a detail level there was a mass grave site at East Harling in Norfork which was considered to be the result of a Viking massacre when discovered. That site is now assessed as a medieval churchyard burial site.
Another site initially assessed as a massacre was in Derbyshire. Found together at that location were 250 remains, primarily men. Obviously a massacre by Vikings, right?
Subsequent research has assessed their ages as from 15 to 45, of noticeably larger size than the males who lived in the area, and with Scandinavian characteristics (whatever that means). In other words, those were Vikings, not victims of the Vikings. This is now assessed as a war grave of Vikings, most likely remains of those who died over a larger area which were gather together in one place for burial.
See page 50 of text for more details.