Conchobar (d. 833), son of Donnchad and king of Tara, belonged to the Clann Cholmáin dynasty of the Southern Uí Néill. His father Donnchad Midi (qv) had retained the kingship of Tara till his death (797). His mother was Fuirseach, daughter of Congal, king of Dál nAraide, and (arising from his father's other marriages and liaisons) he had six or seven half-brothers: Domnall (slain 799), Ailill and Diarmait (both slain 803), Óengus, Folloman (slain 830), Máel-ruanaid (d. 843), and perhaps also Ruaidrí (d. 838), vice-abbot of Clonard and Clonmacnoise. He also had at least three sisters or half-sisters: Eithne (qv) (d. 795), wife of Bran Ardchenn (qv), nominal overking of Leinster; Euginis (d. 803), the wife of Áed Oirdnide (qv), Cenél nÉogain king of Tara; and Gormlaith (d. 861), who married the latter's son Niall Caille (qv) and was the mother of Áed Finnliath (qv). Conchobar's own wife is unrecorded but he had at least two sons, Cathal (d. 843) and Eochacán; he may also have been the father of Bishop Artrí (qv) (d. 833).
The year 797, a disastrous one for Conchobar's family, saw the death of his father Donnchad and of three uncles, Indrechtach, Fínshnechtae, and Diarmait Odar; the defeat of Clann Cholmáin at Druim Ríg (Drumree, Co. Meath); and the devastation of Mide by the powerful Cenél nÉogain dynast Áed Oirdnide, who seized the kingship of Tara and for several years overshadowed a weak and divided Clann Cholmáin. Conchobar clearly resented the promotion of his brother Domnall to the kingship of Mide; he joined with another brother, Ailill, to slay Domnall in 799. In the short term, however, Conchobar's plans to rule Mide were frustrated, as his brother was succeeded by his son Muiredach. On the latter's premature death (802), the kingship of Mide was divided between Conchobar and Ailill by the king of Tara, Áed Oirdnide.
Before long, rivalry within the family led to open warfare. At the battle of Ráith Conaill, Conchobar slew Ailill; another brother, Diarmait, who also claimed the kingship, fell in the same battle or shortly afterwards. Conchobar then reigned alone over Mide, but was clearly dominated by Áed Oirdnide, who several times crossed the midlands to deal with Leinster. In 808 he joined with Muirgius (qv) son of Tommaltach, overking of Connacht, to strike against Áed Oirdnide, but achieved limited success, and the borderlands of Mide were burned in retaliation. After the death of the king of Tara (819), however, Conchobar laid claim to that dignity, and Middle Irish regnal lists accord him a reign of fourteen years. In the 820s he apparently sought to extend his authority over Armagh. If the bishop Artrí, who aspired to the successorship of St Patrick (qv), was Conchobar's son, the battle of Leth Cam (827) may be seen as a failed attempt to promote Southern Uí Néill claims at Armagh in opposition to those of the Cenél nÉogain ruler Niall son of Áed.
Meanwhile, Fedelmid (qv) son of Crimthann had emerged as king of Cashel and was making his presence felt in the midlands. In 827 a rígdál (royal meeting) took place at Birr between Conchobar and Fedelmid; while no certain outcome is recorded, it seems that a modus vivendi was reached between the two. Conchobar then sought to assert his authority over the provinces of Connacht and Leinster. In 829 he raided Connacht, but Fedelmid attacked the same province the following year. On this incursion, Conchobar's brother Folloman was slain by the Munstermen, which may indicate the end of a short-lived alliance. Efforts to subdue the Leinstermen led Conchobar to plunder the Liffey plain in 831. He died in 833, in the same month as Artrí, his putative son. His immediate successor as king of Mide was his brother Máel-ruanaid (d. 843), from whom the later kings of Mide descended.