The basic unit of organisation of Na Fianna Éireann is known as a Slua. A slua normally consists of between 5 and 25 members. A slua may be made up of junior Fianna (under 15) or senior Fianna (over 15) or it may be made up of two sections; one junior and one senior. Junior and senior Fianna are not normally mixed as their functions and activities are somewhat different. Junior Fianna are involved purely in scouting, educational and cultural activities. Seniors on the other hand are also involved in political activities.
Every slua must have three officers – an O/C (Officer Commanding), Adjutant (Second in command) and a Quartermaster (in charge of finance, uniforms and supplies for the slua). The O/C is responsible to his superior officers and to the Árd Choiste (head quarters of Na Fianna) for the correct running of the slua.
The Árd Choiste of Na Fianna is elected every year at the Árd Fheis (national conference). Here delegates from each slua can discuss and decide on Fianna policy. The previous year can be discussed and plans can be made for the next year.