A túath (plural túatha) was a medieval Irish polity smaller than a kingdom. The word is from Old Irish, and is often translated as "people" or "nation". It is cognate with the Welsh and Breton tud (people), the Galician toudo, and the Germanic þeudō (for which see theodiscus).
In ancient Irish terms, a household was reckoned at about 30 people per dwelling. A trícha cét ("thirty hundreds"), was an area comprising 100 dwellings or, roughly, 3,000 people. A túath consisted of a number of allied trícha céta, and therefore referred to no fewer than 6,000 people. Probably a more accurate number for a túath would be no fewer than 9,000 people.
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The organization of túatha is covered to a great extent within the Brehon laws, Irish laws written down in the 7th century, also known as the Fénechas.
The social structure of ancient Irish culture was based around the concept of the fine (plural finte), or family kin-group. All finte descended from a common ancestor out to four generations comprised a social unit known as a dearbhfhine (plural dearbhfhinte). Túatha are often described as petty kingdoms. Owing to the complex and ever-changing political nature of ancient and medieval Ireland, túatha ranged in character from petty kingdoms sovereign in their own right, to areas bound by fealty to much larger "over-kingdoms" such as Connacht or Ulaid. Thus the place of túatha in the socio-political structure of Ireland varied, depending on the power and influence of the individual dynasties at the time.