Celtic Ireland was divided into a number of small kingdoms or tuatha, probably more than a hundred in all each ruled by a king or ri. A number of these rulers were also over-kings, receiving tribute from neighbouring kings. There were also kings of provinces, but the idea of a high king, or ard-ri of all Ireland appears to have been a later invention. Niall of the Nine Hostages, who ruled at Tara at the beginning of the fifth century, may have been the first to claim the title of ard-ri. His descendants, the Ui Neill, were seldom able to press their claim to the high kingship in any material way.

Initially Ireland was divided into the so-called 'five fifths of Ireland'. These corresponded to the present provinces of Ulster, Connacht, Munster and Leinster, except that north Leinster formed the province of Meath around Tara. Later, however, two additional units were formed on the borders of Ulster by the defeat of the Ulaid. These were Aileach and Airgialla.

Each king was elected from a small group of people possessing royal blood, known as the derbfine and comprising the male descendants of a common great-grandfather, four generations in all. Thus a king could be suc ceeded not only by a son or grandson, but by an uncle or a great-nephew.

Beneath the king were the nobles or flaithi. These were warriors and owners of cattle, and had an important role as patrons of the aes dana, the 'men of art', who comprised the learned classes, the poets and musicians, and the skilled craftsmen. Next came the freemen, the tillers of the soil, usually bound by contract to a nobleman. Under this contract, which could be terminated by either party, the nobleman provided protection and lent the freeman cattle to graze his land, receiving in return a rent which might consist of sacks of wheat or malt and possibly a salted pig or a young calf. There were also slaves, probably captured in war, but comparatively little is known about them and they may not have been very numerous.

Celtic society was essentially rural, but the members of a tuath or small kingdom met regularly in an assembly or oenach over which the king presided. This was an occasion also for games and sports. Horse-racing and an equivalent of the modern Irish hurling were common and there were board-games similar to chess or draughts. Feasts took place to the accompaniment of music and story-telling. Wine was imported from the Mediterranean and beer was brewed from Irish barley.