Conn Cétchathach (‘the hundred-battler’), pseudo-historical ancestor of the dynasties which emerged as the Connachta (including Uí Néill) and Airgialla, and forebear of all noble families of Leth Cuinn (the northern half of Ireland), was reputedly a son of Fedelmid Rechtmar son of Tuathal Techtmar. Whether Conn was a pagan deity or a fiction of the early historical period (his name being derived from Connacht – headship – following the adoption of that term by a dynastic group as a ‘quasi-tribal’ name), the sources treat him as an historical character. His father, Fedelmid Rechtmar, supposedly reigned as king of Ireland before the dawn of history, while his mother – named as Úna Oll-chrotach – is anachronistically described as daughter of a king of Lochlann (which in Middle Irish literature means Scandinavia). He is assigned six brothers, the most notable of whom is Eochaid Finn fuath-nAirt, arch-enemy of Conn and of his son Art Óenfher.
According to historical literature, Conn had several wives including Aífe daughter of Ailpín (a British king), mother of his sons Conlae and Art Óenfher and of his daughter Sadb. Significantly, of his other wives, three (Lendabair, Lann, and Eithne Thóebfhota – featured in the tale ‘Echtra Airt’) have strong Leinster connections. The first is alternately represented as a former wife, or daughter, of the Laigin ancestor-figure Cathaír Már (qv), the second is said to have been a daughter of Cathaír, and the third (mother of his son Crínna and daughter Sárait) a daughter of the prehistoric Laigin king, Crimthann Cas. Equally significant are the links forged between Conn and the ruling elites of the Éoganachta and the Érainn – prominent in Munster – through the marriages claimed for his family. His daughter Sadb was allegedly the wife of Ailill Ólom (qv), an ancestor figure of the Munstermen, and mother of his nine sons including Éogan Már (qv) – sometimes identified with Mug-Nuadat – from whom the Éoganacht dynasties claimed descent. She is also said to have been the mother of Lugaid son of Conn of the Érainn. Another daughter, Sárait, supposedly married Conaire Már son of Mog-Láma, an ancestor of various Érainn peoples, and was the mother of three sons, each named Cairpre.
Conn features as ‘king of Ireland’ in historical literature from the Old Irish period onwards. It is implied by the regnal poem ‘Baile Chuinn’, and expressly claimed in ‘Baile in Scáil’ and other Middle Irish king-lists, that he held the kingship of Tara and reigned for twenty, or even fifty-three, years. Medieval historical tradition places him at Cenannas (Kells, Co. Meath), while he awaited his ‘call’ to Tara, to succeed Cathaír Már as king. According to the Leinster poem ‘Timna Cathaír Máir’ (‘The testament of Cathaír Már’), he defeated and slew his father-in-law at Mag Ága (near Teltown, Co. Meath).
Conn's reign was depicted as a time of peace and unrivalled prosperity, despite his being obliged to share sovereignty with Éogan Már and partition the country along a line from Galway to Dublin to form Leth Cuinn (Conn's half) and Leth Moga (Mogh's half). Although one account claims that Conn defeated Éogan Már at Mag Léna (Moylen Heath, parish of Kilbride, barony of Ballycowan, Co. Offaly) to become sole ‘king of Ireland’, another version has him expelled from Tara by a Leinster rival, Eochaid son of Erc – enduring exile for seven years before he overcame the latter. Conn's ultimate demise came about, it is said, through the treachery of his brother Eochaid Finn, who plotted with an Ulster king, Tipraite, to assassinate him. Some Middle Irish lists have Conn followed in the kingship by his son-in-law Conaire, and the latter in turn by Art.