The word “outlaw” conjures up images of wild and woolly men, with no allegiance and nothing to lose. In the Celtic world, that’s partly accurate. You couldn’t get much lower than being an outlaw (except maybe being a slave). But outlaws (men and women) had much more in common with the highest Celtic class, the warriors, than you might think.
Simply put, outlaws were exiles from their tribe. You could become an outlaw for many reasons: going against the judgement of Druid (whose word was always final), breaking a tribal law or even not being able to pay a fine. In the later case, you would then plunder a neighboring tribe to raise the funds you needed to be reinstated (logical, right?). Exiled nobles often raised an army and fought their way back into society. But others did not have such options available to them.
Outlaws lived in an area of wilderness between the tribes that was vast and empty of what we would consider organized civilization. But they usually had at least a tacit alliance with a local tribe, for both their protection and that of the tribe. In places where this agreement occurred, the outlaws could be called upon to fight with and defend the tribe, if needed.
But they could also be called upon to do the tribe’s dirty work. Since the land and goods of a monarch were inherited by kin, not necessarily the chosen successor, murder and revenge were common to try to gain power. Nobles often employed bands of outlaws as mercenaries to raid or kill their neighbors. Because they were “outside of the law,” outlaws could not be punished under the law by those they harmed. If the wronged party was to have revenge, they had to track down the mercenaries in the vast, lonely wilderness, a place which the outlaws knew better than their pursuers.
Because they lived in the wilderness, outlaws were also thought to have special powers over Otherworldly nature creatures, including the Sidhe (faeries) and elves. In Irish mythology, there was a special band of outlaws called the Fianna, an elite group of outlaw warriors, who had no clan and no specific allegiance, but who often passed between this world and the Otherworld.
In case you’re interested, most of this information came from Who Were the Celts by Kevin Duffy and Women in Celtic Law and Culture by Jack George Thompson, both of which are mentioned on my research page.