Na Fianna Éireann, known as the Fianna, is an Irish nationalist youth organisation founded by Constance Markievicz in 1909, with later help from Bulmer Hobson. Fianna members were involved in setting up the Irish Volunteers, and had their own circle of the Irish Republican Brotherhood (IRB). They took part in the 1914 Howth gun-running and in the 1916 Easter Rising. They were active in the War of Independence and many took the anti-Treaty side in the Civil War.

Fianna were small warrior-hunter bands in Gaelic Ireland during the Iron Age and early Middle Ages. A fian was made up of freeborn young males, often from the Gaelic nobility of Ireland, "who had left fosterage but had not yet inherited the property needed to settle down as full landowning members of the túath". For most of the year they lived in the wild, hunting, cattle raiding other Irish clans, training, and fighting as mercenaries. Scholars believe the fian was a rite of passage into manhood, and have linked fianna with similar young warrior bands in other early European cultures.