That subject should be "Irish raids.
I'm reading RPC Hanson's _St. Patrick, His Origins and Career_. The first
chapter is a pretty good review of the history of 4th - th century Britain
After the Romans left (and probably while they were there, too) there was the
constant threat of raids from the Irish from the west and Angles and Saxons
from the east. Does anyone know why these folks were so bent on raiding and
pillaging? Was it a need for more land? (During this time the Irish
established the colony of Dalriada) Was it a macho thing?
Were they just doing it for grins?