The Ui Neill were the most prominent political dynasty in Ireland from the seventh to the late tenth century. The annals for this period contain copious references to the dynasty that allow historians to reconstruct their story in reasonable detail. The Ui Neill claimed descent from Niall Noigiallach who lived in the fourth or fifth century. Further back, the Ui Neill were said to be a branch of the Connachta, descended from Conn Cetchathach ("Conn of the Hundred Battles").
The origins of the dynasty pre-date recorded history and are shrouded in obscurity. T. F. O’Rahilly believed that they emerged from the east midlands. However, it is much more likely that they originated in northeastern Connacht.
The Ui Neill comprised a number of distinct dynastic groupings each of which claimed descent from a different son of Niall. J. V. Kelleher argued that the Ui Neill were a federation of tribes who contrived a common descent from Niall Noigiallach. It was certainly the case that the number of sons attributed to Niall grew over the centuries as other tribal groupings fell under Ui Neill control and assumed Ui Neill identity by crediting their founding ancestor figure as a son of Niall. However, it seems fairly certain that there was an early core Ui Neill grouping onto which these later accretions were grafted. This core group probably included the descendants of Loegaire, Coirpre, Fiachu, and Conall (Mac Shamhrain has argued that the two putative sons of Niall named Conall, namely Conall Gulban and Conall Cremthainne, were one and the same person). The full flowering of Ui Neill expansion saw no fewer than fourteen sons being claimed for Niall.
By the sixth century, the basic dynastic structure of the Ui Neill, which was to characterize the history of the dynasty for centuries to come, had begun to take shape. Cenel Conaill, Cenel nEogain, and the relatively obscure Cenel nEnnai were settled in northwest Ulster (see Ui Neill, Northern), Cenel Coirpri to the south in northeastern Connacht, Cenel Loegaire in various locations from Loch Erne to the Slieve Bloom mountains, the descendants of Conall Cremthainne (later to emerge as Clann Cholmain and Sfl nAedo Slaine) in the east midlands, Cenel Fiachach in the center of the country near Uisnech (see Ui Neill, Nouthern). Ui Neill dominance of the northern half of Ireland gave rise to that area’s being known as Leth Cuinn ("Conn’s Half").
From about the mid-seventh century onward, the Ui Neill had assumed proprietorial rights to the ancient sacral kingship of Tara, and indeed the term r( Temro ("king of Tara") was to become synonymous with the over kingship of the Ui Neill dynasty. The balance of power between the northern and southern branches of the Ui Neill was maintained from the mid-eighth to the late tenth century by means of an arrangement whereby, with only one exception, the kingship of Tara alternated between a member of Cenel nEogain (who had emerged as the dominant grouping among the Northern Ui Neill) and the Clann Cholmain (the strongest branch of the Southern Ui Neill).
Various Ui Neill kings styled themselves "king of Ireland" from at least the time of Domnall mac Aedo of Cenel Conaill (d. 642) onward. However, it was not until the ninth and tenth centuries, during the reigns of Mael-Sechnaill I mac Maele-ruanaid (d. 862), and a number of his successors, that the U Neill achieved anything approaching dominance over the entire country.
The rise of the W Neill to prominence, and the subsequent maintenance of that dominance, owed much to their relationship with the church. From the seventh century onward, they were closely allied to Armagh, which was at that time asserting its own claims to primacy over the church in Ireland. Branches of the U Neill also had close associations with the various churches of the Columban federation; St. Colum Cille (d. 597), belonged to Cenel Conaill of the Northern U Neill. Later, the W Neill, and especially the Clann Cholmain, were to forge strong links to the major monastic foundation of Clonmacnoise, where a number of the antiquities from the tenth and eleventh centuries bear testimony to the close association between church and dynasty.
The decline of the U Neill dynasty in the late tenth century was due in large measure to the emergence of the Dal Cais dynasty in Munster—a dynasty which under Brian Boruma (d. 1014), his grandson Tairrdelbach (d. 1086), and his great-grandson, Muirchertach Ua Briain (d. 1119) demonstrated the ambition and vigor to lay claim to the overlordship of Ireland.
