The Túath was the minimal polity of ancient Ireland and it consisted of a Rí (king) and his people and their territory. Among the British a similar leader was sometimes termed a Brenin which Charles-Edwards considers a term of huge antiquity. There were other terms too notably Tigern.
It seems there were 150 or so Túatha (plural) in Ireland which gives you an idea of the scale we are talking about. The minimal polity was in practice a component part of a much bigger political entity; in Ireland 4 or possibly sometimes 5 of these bigger entities existed.
External politics worked like this:
The Rí had to decide which of his neighbouring Túatha might or could be brought to clientship and if any of them could make him become a client. He then acted accordingly. A Rí who could command other kings was acknowledged as a Wledig in Britain.
The most famous example is Macsen Wledig (known to his fellow Romans and sometime subjects as Flavius Magnus Maximus Augustus) who figures as the founder of so many British and some Irish dynasties.
There were others too and we can be quite sure that the normal state of affairs was for smaller kings to be commanded by more powerful monarchs. Thus the micro - polities advocated by some scholars were real enough but crucially,and this is sometimes missed, they formed the component parts of bigger more powerful polities.
Internal politics worked more or less like this:
The members of the ruling clan had to decide which individuals from which segments of their agnatic kin group were best suited to be king.
Aristocrats had to decide which candidate from which segment of the ruling clan they would support.
Free men of the Túath had to choose from the many aristocrats the man whom they would support.
These were all individual decisions to be carefully calculated.
The later Welsh saying ‘Me against my brother, my brother and me against my cousins and my cousins and my brother and I against the world” illustrates the competitiveness and resilience of the system perfectly.
This as far as we can tell was the system across the Celtic world and in considering the sources for At the Ends of Empire I keep it in mind.