The Fothairt were a Medieval Irish people based in Leinster.
The Fothairt are referenced in Leabhar Ua Maine. Their genealogies are preserved in Leabhar na nGenealach, the Great Book of Lecan, Bodleian Library, MS Rawlinson B 502, and/or Bodleian Library, MS Rawlinson B 512.
Dáibhí Ó Cróinín writes that "The Fothairt were the original population group around the site of Bridgit's monastery at Kildare" (p. 197), further stating that the Uí Ercáin, a branch of the Uí Meic Cruaich (a sub-sept of the Fothairt) "are remembered in the 'Vita Tripartia' as having been specially favoured by Patrick, who blessed them (Dobert Pátraic bendachtain ... for Uu hErcá huili) and their king, Fergnae mac Cobthaig, who is also mentioned in the life of Fintan/Munnu of Tagmon, where he encounters the saint in campo Lyffi ('in the Liffy plain')." (p. 197).
The kingship of Leinster was held from the mid-8th century to 1042 by the Uí Dúnlainge, who bolstered their early gains "by means of political marriage with Sárnat, daughter of Eochu mac Baíth of the Fothairt" (p. 197). A branch of the Forthairt, the Uí Brigti, explicitly claimed a connection with Bridgit (p. 198). In addition, two kings of Forthairt, Fergus mac Móenaig and Dub dá Chrich mac aui Cellaig mec Triein, "as well as 'many others, omitted for the sake of brevity'", were killed at the battle of Ath Senaig (Ballyshannon, near Kilcullen, County Kildare) in 738 as supporters of King Áed of Leinster (p. 200).
Despite their political eclipse, the Fothairt "continued to supply leading clergy to Kildare. To Ui Chúlduib ... belonged the two abbesses, Muirenn (d. 918) and Eithne (d. 1016); to the obscure Forthair Airbrech belonged the earlier abess, Sebdann (d. 732) and another Kildare ecclesiastic (d. 750); while two further abbesses, Coblaith (d. 916) and Muirenn (d. 964), belonged to an ecclesiastical branch of Forthairt Fea. And it is highly probable that many other Kildare clerics, whose origins cannot be established with certainty, belonged to Fotharta." (p. 586, NHI) Indeed, the Uí Ercáin were noted in the 'Tripartite Life' as being blessed by Saint Patrick, who promised them that not only would they never be subjects of an "outside" king, but that "they [would] have their own brithemnas, capacity of judging and being judged, in their own territory. *(p. 873, NHI)
All quotes from the Annals of the Four Masters, unless otherwise stated.