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), Galician toudo, and with 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.
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. Due 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 describing their place in the socio-political structure of Ireland is varied depending on the power and influence of the individual dynasties at the time.
- Osraige - túath that later became the kingdom of the same name in the Christian era
- Dál Riata - the túath that became a confederation of túatha and eventually settled in Alba, creating the modern nation of Scotland
- Clandonnell, Glenconkeyne, Killetra, Melanagh, Tarraghter, and Tomlagh, which all once formed the ancient territory of Loughinsholin
- Colonisation under early kings of Tara, Eoin Mac Neill, Journal of the Galway Archaeological and Historical Society, volume 16, pp. 101–124, 1935
- Corpus genealogiarum Hibernia, i, M.A. O'Brien, Dublin, 1962
- Early Irish Society Francis John Byrne, in The Course of Irish History, ed. T.W. Moody and F.X. Martin, pp. 43–60, Cork, 1967
- Hui Failgi relations with the Ui Neill in the century after the loss of the plain of Mide, A. Smyth, Etudes Celtic 14:2, pp. 502–23
- Tribes and Tribalism in early Ireland, Francis John Byrne, Eiru 22, 1971, pp. 128–166.
- Origins of the Eoghnachta, David Sproule, Eiru 35, pp. 31–37, 1974
- Some Early Connacht Population-Groups, Nollaig O Muraile, in Seanchas:Studies in Early and Medieval Irish Archaeology, History and Literature in Honour of Francis John Byrne, pp. 161–177, ed. Alfred P. Smyth, Four Courts Press, Dublin, 2000
- The Airgialla Charter Poem:The Political Context, Edel Bhreathnach, in The Kingship and Landscape of Tara, ed. Edel Bhreathnach, pp. 95–100, 2005