A fian (plural fianna) was an early Irish männerbund: a small, semi-independent group of unmarried young men who lived apart from society in the forests as mercenaries, bandits and hunters, and could be called upon by kings in times of war. A member of a fian was called a féinnid; the leader of a fian was a rígféinnid (literally "king-féinnid"). The typical féinnid was a free-born young man who had left fosterage (at around fourteen years old), but had not yet inherited the property that would enable him to marry and settle down as a member of the túath or petty kingdom, although female féinnidi are also known in legend.
Geoffrey Keating, in his 17th century Foras Feasa ar Éirinn, says that during the winter the fianna were quartered and fed by the nobility, during which time they would keep order on their behalf, but during the summer, from Beltaine to Samhain, they were obliged to live by hunting for food and for pelts to sell. The Foras Feasa is more a compilation of traditions than a reliable history, but in this case scholars point to references in early Irish poetry and the existence of a closed hunting season for deer and wild boar between Samhain and Beltaine in medieval Scotland as corroboration.
The institition appears to date to pre-Christian times, and attracted strong clerical disapproval in the pre-Norman period. By about the 12th century it seems to have died out. The fianna which appear in post-Norman literature, in particular those led by Fionn mac Cumhaill in the stories of the Fenian Cycle, are depicted as a standing army in the service of the High King, although they are still depicted as living apart from society and surviving by hunting.