While the Late Roman/Early-to-Mid-Medeival Irish are most widely known for their proselytizing monks and scholars (Saints Patrick and Columba, for example), Irish mercenaries were far more plentiful, typically organized into heterogenous units, widely employed by folks wishing to add weight to an existing military or even simply to hire a formidable force without the bother and expense of maintaining one's own military. Fighting in someone else's army has a long and revered tradition among the Irish, the career for centuries being, apart from the clergy, about the only way an Irishman might hope to better his lot while remaining "within the law". Not all Irish of ambition were overly concerned about becoming outlaws, though; brigandry and piracy were careers relatively well-populated by the Irish as well.
And that prolly explains a lot