Ireland at this period was rules by at least 150 Tuath Kings, or Ri Tuaithe. These were theoretically under the rule of Ri Coiced, or Kings of a Fifth (Coiced). There were also Rui Ri who were Tuath Kings, that had other Tuath Kings as vassals.
The Tuath was based around a single clan containing several family groups. These were either doer-chele, bound clients, or soer-chele free clients who were the noble elites of the Tuath. A Rui Ri or Ri Ruirech would ensure loyalty by taking hostages, usually the sons of lesser kings. Each Ri was expected to lead the warriors of their Tuath in support of his overlord during wars.
By the third century an army of full time warriors was established. This was the Fiana under a Ri Feinnidh, or commander. This army was divided into tricha cets, thirty-hundreds. During the time of Cormac there were three tricha cets in peace time and 7 during times of war. Tuath warriors were part time soldiers and could not be called to serve during spring planting or the fall harvest. This made the Fiana a necessity for the fighting of any extended war. The Irish warriors of the period wore very little armour and fought with spears and short sword.
The Irish armies of the period depended on speedy attacks and ambushes to win their battles. They would lay in wait in woods to ambush their enemies, using plashings (undergrowth and brush woven together to make a barricade) for cover. They would block the trails and rain javelins upon their enemies from behind these plashings. The Irish also liked to fight in marshes and bogs, using ditches and the marshy terrain to slow their enemies while they rained death upon them with their javelins.