Congalach Cnogba (d. 956), son of Máel-mithig and king of Tara, belonged to Síl nÁedo Sláine, a Southern Uí Néill dynasty. His achievement is noteworthy not only in that he was the last of his dynasty to reign as overking of Uí Néill but also because he claimed sway over Leinster and Munster for a period in the early 950s. Aside from his inclusion in the Middle Irish Tara king-lists, he is one of a small number of rulers styled rí Érenn (king of Ireland) in the original hand of the Annals of Ulster. Congalach owes his sobriquet to the location of his dynastic segment – the lineage of Uí Chonaing – at Cnogba (Knowth, Co. Meath). His father, Máel-mithig son of Flannacán, died (919) as king of Brega (east Co. Meath and north Co. Dublin); his mother was Lígach, a daughter of the Clann Cholmáin king of Tara, Flann Sinna (qv) son of Máel-Sechnaill (qv) (d. 862). He had at least one brother, Áed (d. 965).
An apparent pattern of alliances with other Uí Néill dynasties, secured through his mother's connections and through those of his grandmother Eithne (a daughter of the Cenél nÉogain king of Tara, Áed Finnliath (qv)), was continued by Congalach's judicious marriages. His choice of marriage partners suggests a desire to maintain a certain cohesion within Uí Néill. He married Deichter, daughter of Beollán son of Ciarmac, king of Southern Brega, who belonged to Clann Chernaig Shotail, a rival lineage within Síl nÁedo Sláine. Deichter bore him a son, Domnall (d. 976). Another wife, Eithne of the northern-based Cenél nÉogain, was the mother of his son Muirchertach (d. 964). In addition, he pursued alliances with southern dynasties: he subsequently married Cacht, of the ruling lineage of Osraige, and at a later date his daughter Derbail was married into the Uí Fhailge dynasty of Leinster. He had at least two other sons whose mothers are not recorded, Conaing, and an unidentified son who in turn was the father of another Muirchertach (d. 995).
Congalach was preceded as king of Brega by his cousin Flann (d. 932) son of Máel-Finnia, but it is not clear if he succeeded his cousin directly. Certainly, he had achieved some prominence by 939, when he defeated the local ruler of Gailenga at Áth dá Loarc (near Kells, Co. Meath). He was ruthless in suppressing rivals within his own dynasty: in 942, he killed two distant cousins, Ailpín and Congalach, sons of Lorcán son of Dúnchad (incorrectly ‘Donnchad’ in AU). In the years that followed, Congalach saw increasing scope for political advancement. The kings of Ailech and of Tara, Muirchertach na Cochall Craicinn (qv) of Cenél nÉogain, and Donnchad Donn (qv) son of Flann Sinna of Clann Cholmáin, died in 943 and 944 respectively. Congalach, whose mother was a sister of Donnchad, was well placed to advance his claims to the kingship of Tara, although Ruaidrí Ua Canannáin (qv) of Cenél Conaill soon emerged as a strong challenger.
Meanwhile, Congalach skilfully exploited the delicate balance of power between the Leinstermen and the Dublin Norsemen. Shortly after staking his claims to overkingship of Uí Néill in 944, Congalach allied himself with the Leinstermen and sacked Dublin. The following year, however, he joined forces with Amlaíb (Óláfr) Cuarán (qv), Hiberno-Norse king of York, who supported him in blocking an advance into the midlands by Ua Canannáin. The latter returned in 947 and managed to defeat Congalach and his allies at Slane, but was unable to follow up on this initiative because of dissension within the Northern Uí Néill. Shortly afterwards, Congalach took steps to strengthen his alliance with Amlaíb Cuarán by killing his sub-king Blacair (qv), who had assumed control of Dublin in defiance of his overlord. This intervention turned out to be fortunate for Congalach: when a renewed invasion of Brega by Ua Canannáin brought him to the brink of political eclipse in 950, the Norsemen of Dublin saved his kingship by defeating and killing his arch-rival.
It seems that by 951 Congalach had recovered from his political difficulties; that year his Hiberno-Norse allies plundered Kells, while he launched a major effort against Dál Cais. Previously, he had invaded Thomond and defeated the Dál Cais king, Cennétig (qv) son of Lorcán, slaying two of his sons. He now placed an Uí Néill fleet on Lough Derg and took hostages from Munster. There may nonetheless be truth in the claim that the Éoganacht overking of Munster, Cellachán Caisil (qv), plundered the borders of Meath in response. The early 950s represent the high-point of Congalach's career; he was pre-eminent among the kings of the Uí Néill and overshadowed Munster and Leinster. It was perhaps during this period that the poem on the ‘Dindshenchas’ (topographical lore) of Brug na Bóinne was addressed to him. As the middle of the decade approached, however, difficulties began to accumulate. His situation was, doubtless, not helped by strained relationships with the Columban familia. The community of Kells, having enjoyed the patronage of his Clann Cholmáin predecessors, certainly had grounds for resenting a king who dispensed favour to Clonard, who frustrated Cenél Conaill ambitions, and who probably bore some responsibility – even indirectly – for the Norse raid on their foundation in 951. A notable Kells document, the Irish Life of Adomnán (qv) (d. 704), apparently features Congalach in the guise of Congal son of Fergus, for whom a short reign and a bloody end is prophesied by the saintly abbot of Iona.
In 954 Congalach's home-territory of Brega was ravaged by Domnall Ua Néill (qv) of Cenél nÉogain, who had by this time emerged as his principal rival. When Congalach campaigned to secure the hostages of Connacht he found himself in competition with Domnall, who established a better claim to dominance over the western province. In 956, he again turned his attentions southwards; he subdued Leinster, and proceeded to celebrate the Óenach Colmáin (fair of Colmán, on the Liffey plain), a prerogative of the Leinster overking. Perhaps his blatant disregard for local political sensitivities stirred resentment among dynasties which had earlier tendered submission. In any event, the north Leinstermen joined in revolt with his erstwhile Hiberno-Norse allies; he was ambushed at Tech Giugrann, near Dublin, and killed along with a number of his retainers.
The claim of Congalach's dynasty to the kingship of Tara, or even to the less prestigious kingship of Uisnech, died with him. His brother Áed, styled rígdamna Breg (eligible for the kingship of Brega), died in 965 on pilgrimage at St Andrews in Scotland. A year earlier, his son Muirchertach who was certainly prominent amongst the Southern Uí Néill and was styled rígdamna Temrach (eligible for the kingship of Tara), was killed by his brother Domnall, perhaps through some misunderstanding. This Domnall died as king of Brega in 976, but Congalach's other sons seem to have achieved no great political distinction. The decades that followed witnessed considerable strife in Brega. Ultimately, the line of Ua Cellaig Breg (descended from Cellach, an uncle of Congalach) emerged as local rulers.