Ua Conchobair, Tadc in Eich Gil (d. 1030), a son of Cathal and overking of Connacht, belonged to the dynasty of Síl Muiredaig. In later sources he is called in Eich Gil (of the white steed) to distinguish him from the earlier Tadc in Túir (of the tower; d. 956) whose father was also named Cathal. The father of the later Tadc, Cathal (qv) son of Conchobar (qv) from whom descended the family line of Ua Conchobair, died as overking of Connacht in 1010. His mother was Sadb, daughter of Áed grandson of Cellach king of Uí Maine; he apparently had two (or three) brothers (or half-brothers), including Domnall (d. 1013), dubbed in Catt (the cat), and Brian (slain 1028). He himself married Derbfhorgaill, daughter of Tadc son of Gilla-Pátraic king of Osraige; she was the mother of his son Áed in Gaí Bernaig Ua Conchobair (qv), but reportedly she had nine other children through later marriages to husbands that included Tairdelbach Ua Briain (qv) (d. 1086).
Tadc succeeded his father directly in the overkingship in 1010; the regnal list accords him a reign of twenty years. Early in his reign he faced a threat from his neighbours to the north, Uí Briúin Bréifne and Cenél Conaill. Hostilities with these dynasties in 1013–14 resulted in an incursion on Mag Aí (the plain around Cruachu, Co. Roscommon) which cost the life of Domnall in Catt. In retaliation, the following year Tadc slew the king of Bréifne, Áed grandson of Ruarc, father of Art and grandfather of Áed Ua Ruairc (qv). He coped effectively with pressures from within Connacht; in 1023 he slew Domnall grandson of Egra, local king of Luigne. Coming under increasing pressure from the powerful overking of Munster, Donnchad (qv) son of Brian, Tadc yielded hostages at Cruachu (1025). Whether he lent support to the latter, or ceded control of his southern realms to him, is not clear, but in 1027 a south Connacht dynast was slain on a Munster hosting in Osraige.
In any event, Tadc seems to have intervened in the politics of Mide, with fatal consequences for himself. He was slain in 1030 by a Clann Cholmáin dynast, one Máel-Sechnaill grandson of Máel-ruanaid, at the same time as Domnall in Got (the stammerer) who claimed the kingship of Mide; the Annals of Inisfallen, however, attribute his death to the Connachta themselves. His death was followed by a period of instability during which provincial suzerainty was claimed by Art, the father of Áed Ua Ruairc. Tadc's son Áed emerged to take the overkingship in 1046.