Fianna

In early Ireland, fianna (singular fian) were small, semi-independent warrior bands who lived apart from society in the forests as mercenaries, bandits and hunters, but could be called upon by kings in times of war. They appear in Irish mythology, most notably in the stories of the Fenian Cycle, where they are led by Fionn mac Cumhaill. In more recent history, the name Fianna Éireann has been used by a number of Irish Republican scouting organisations.
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[Celtic] Also known as the Champions of the Red Branch, Fianna Éireann, and the Fenians. The Fianna were a legendary army of Irish warriors serving under the Ard Righ, or "High King", of Ireland. Many of their exploits are documented throughout the Fenian Cycle which took place circa the 3rd century CE. One of two Celtic classes of fighters, they lived in the borderlands, fighting in large groups. They obeyed only their own laws and those of the High King. Their last and greatest leader was Finn mac Cumhail, also know as Finn mac Cool, who was later glorified as an Irish hero.