Early Celtic society was divided into tribal groups called tuatha. A tuath (singular) was like a kingdom in miniature, a family group that usually claimed descent from a common ancestor. Most tuaths operated as independent entities, but many came together as part of larger kingdoms under a central ruler. This was the beginning of the succession of chieftains and high kings.

Each tuath was headed by a king (or, sometimes, a queen) who usually claimed descent from one or more tribal ancestral deities. In times of peace, the king was both ruler and an administrator of justice; in times of conflict, he was a warlord. The king was vested in his office through his symbolic marriage with the land, which was personified as a goddess. This rite of investiture ensured the fertility of the land and is echoed endlessly in mythological tales. The legendary Queen Medb of the Ulster Cycle is a personification of the sovereignty of the land, queen to nine kings who ruled only with her consent — a metaphor for the true source of a king's power and his true responsibilities.

The Celtic system of rulership was bound by a strict code of justice, given that the prosperity of a king's tribe depended upon his fairness and honesty. This was a practicality, as the king also acted as landlord to his people and provided them both protection and grazing land. In later times, these lesser kings themselves swore allegiance to an over-king, who traded military protection in return for tributes of food, supplies, or treasure. A Celtic king was often elected to his position, and if he did not fulfill the role as expected, he could be replaced.

Under the king were the nobles or flaithi — warriors, artisans, lawyers, poets, and other skilled citizens. Under the noble class were the freemen who kept the flocks, tilled the soil, and paid rent to the nobles. Each class contained subdivisions, each with its own rights and responsibilities.