Diarmait (d. 565), son of Cerball and king of Tara, was an early representative of Uí Néill and was claimed as common ancestor of the Síl nÁedo Sláine and Clann Cholmáin dynasties. His father is named in Adomnán's ‘Vita Columbae’ and in the Annals of Ulster, and is presumably the Cerball son of Fergus son of Conall Cremthainne (qv) featured in the ramifications of the genealogies. Representation of Diarmait in the Uí Néill pedigrees as a son of Fergus Cerrbél seemingly arises from confusion of his father and grandfather. Having adjusted his pedigree accordingly, it emerges that Diarmait belonged to the same generation as Colum Cille (qv), whose grandfather was one Fergus son of Conall Gulban (qv). Moreover, there are strong indications that the two Conalls, both allegedly sons of Niall Noígiallach (qv), are duplicates. This being so, the relationship between the dynasties of Síl nÁedo Sláine, Clann Cholmáin, and Cenél Conaill and, on a personal level, between Diarmait and Colum Cille was closer than formerly realised. Diarmait's mother is named as Corbach, daughter of Maine Laignech. The Leinster connection implied in his maternal grandfather's sobriquet may be significant, given that Diarmait later achieved political power in the midlands. He had at least two brothers, Illann and Maine (slain 538), and – according to some accounts – a foster-brother (or uterine brother?) named Máel-mór (Máel-mórdai) son of Airgetán.
It is probable that Uí Néill, in Diarmait's day, retained much of its political influence west of the Shannon. This, in all likelihood, underlies the ‘Banshenchas’ (lore of women) claim of marriage links on his part with the Conmaicne, a population group which in the historical period was scattered across Connacht (leaving its name on the district of Connemara, ‘Conmaicne Mara’). There is considerable confusion in the record, but an alliance is indicated with the Conmaicne of Cúl Talaid (near Lough Mask). It said that women of that lineage, Brea (or Breaca) daughter of Colmán and Eithne daughter of Brénainn Dall (or Lasair daughter of Eachtan), were the respective mothers of Diarmait's sons Colmán Már (qv) and Colmán Bec (qv). However, it seems that the brothers Colmán represent another case of duplication – which leaves it likely that the Conmaicne marriage-alliance has been doubled up. There is a possibility that Diarmait named his son Col[u]m[b]án after his cousin Colum Cille, which would have been a very early adoption of the name Columba for secular usage. Another wife claimed for Diarmait was Mugain, daughter of Cúcraide son of Duí of Éoganacht Chaisil; she was the mother of his son Áed Sláine (qv). Bressal, another alleged son of Diarmait, features in Middle Irish literature.
It can be inferred from Adomnán (qv) that Diarmait ultimately ruled over other dynasties besides Uí Néill. Perhaps his initial kingship was amongst the Connachta; certainly, his floruit coincides with an early expansion of Uí Néill power. The death of his brother Maine in the battle of Cláenloch (near Gort, Co. Galway?) is dated to 538. From about this time, Diarmait was emerging as a powerful figure among the Uí Néill. In 544 the reigning king of Tara, Tuathal Máelgarb (qv), was slain. According to a gloss in the Annals of Ulster he was killed by Diarmait's foster-brother Máel-mórdai. Be that as it may, the years that followed saw Diarmait strive to secure his hold on the kingship of Tara, a prize which till a short time before had been claimed by the Leinstermen and their allies, and was still contested by the Dál nAraide overkings of Ulster, whose sway extended as far south as the Boyne. A gradual extension of his dynasty's power in the midlands is implied by the death-tale of his son Bressal (‘Aided Bressail’), which has Diarmait based at Kells.
Eventually, Diarmait was successful in his goal; the Old-Irish regnal poem ‘Baile Chuinn’ includes him among the kings of Tara. The ‘Vita Columbae’ singles him out as rex totius Hiberniae (king of all Ireland), which may well indicate that he was the first of his dynasty to achieve such distinction. Equally noteworthy is the insistence of the same source that he was ‘ordained by God’. Almost certainly, this is an anachronistic claim regarding regnal investiture, but is intended to emphasise the Christian dimension of his achievement. However, the survival of historical tradition associating Diarmait with pagan custom, as is the case with his older contemporary Éogan Bél (qv), is equally strong. According to the Annals of Ulster (558), Diarmait celebrated the Feis Temro (feast of Tara) which, in view of its clearly pre-Christian character, would hardly have met with the approval of the Columban community. It is significant that the feis was held some years into his reign (fourteen, it seems: Middle-Irish king-lists claim that his reign totalled twenty-one or twenty-two years), which suggests that it did not celebrate his initiation as king, but marked a change in the character of his kingship.
As perhaps befits a king who flourished at a time when pagan values were already being superseded by Christianity, Diarmait is represented as an ambivalent figure in literature, including hagiography. He is said to have been a personal friend of Ciarán (qv) of Clonmacnoise and to have helped the latter in founding his monastery. Subsequently, when Diarmait celebrated the Óenach Tailten (fair of Tailtiu; Teltown, Co. Meath), Ciarán attended and worked a miracle. As against this, Diarmait is said to have offended Ruadán (qv) of Lothra by slaying a Connacht dynast within his sanctuary, prompting that saint to place a solemn curse on Tara so that it thereafter became a desolate site. This tradition presumably reflects the ultimate abandonment of a pagan sacral kingship.
Diarmait's political achievement is also qualified in several respects. While his reign is characterised by expansion of Connachta–Uí Néill sovereignty in the midlands, there appears to have been some erosion of his dynasty's power in the west. The emergence of the Uí Briúin dynasty of Connacht under Eochu Tírmchárna and his son Áed Abrat (qv) may be a by-product of this shift in Uí Néill ambitions. By the same token, the hold of Diarmait's dynasty on the kingship of Tara and on the midland realms was, and probably long remained, insecure. Although the annal entry concerning the killing of his son Colmán Már by the Dál nAraide c.558 seems dubious, in view of the probability mentioned above that the brothers Colmán were duplicates, it is not unlikely that Uí Néill earned the enmity of that north-eastern dynasty. It may be noted that Diarmait's eventual fall is ascribed to a Dál nAraide king.
Meanwhile, his expansionist aims likewise provoked opposition from within Uí Néill. In 561 he suffered a signal defeat at Cúl Dreimne (near Drumcliff, Co. Sligo) from a Cenél Conaill faction led by Báetán (qv) son of Ninnid in alliance with Cenél nÉogain under the brothers Forggus and Domnall sons of Muirchertach/Mac Ercae (qv), and with the Uí Briúin king Áed Abrat. The context of this battle has been obscured in Columban hagiography, perhaps deliberately so to conceal whatever role the saint may have actually played in the events which led to his ecclesiastical trial and self-exile. There may be truth in the account in the Annals of Tigernach (Tigernach Ua Bráein (qv)) that Diarmait slew a son of the Connacht king while the latter was under Colum Cille's protection. In any event, the version of events in ‘Vita Columbae’, which alleges that Diarmait pronounced a judgement against Colum Cille because he copied a book without permission and so precipitated the battle, seems less plausible.
The outcome of the battle of Cúl Dreimne clearly weakened Diarmait's hold on the midlands. The following year, he was defeated at Cúl Uinnsen by Áed son of Brénainn king of Tethbae, one of his own vassals. He was slain (565) by a Dál nAraide king, Áed Dub (qv) son of Suibne Araide. Middle Irish tradition places the killing at the Dál nAraide stronghold of Ráith Becc (Co. Antrim), although death in the Boyne valley, where the interests of the rival dynasties collided, would seem a more appropriate end for Diarmait. Memories of the sacral kingship of Tara, with which he had associated himself, may well have prompted the tale ‘Aided Diarmata’, in which the anti-hero perishes amid prophesies of doom, his end illustrating the characteristic ‘threefold death’ motif. The fate of his killer suggests that the Columban community did not assign Diarmait ultimate blame for the exile of Colum Cille, whatever the precise circumstances preceding Cúl Dreimne and the saint's ecclesiastical trial; the unlucky Áed Dub is bitterly condemned in ‘Vita Columbae’, and a suitably violent end is prophesied for him.
Some confusion in the Uí Néill regnal sucession apparently followed the death of Diarmait. According to Middle Irish king-lists, the brothers Forggus and Domnall held the kingship of Tara for a year, and were followed by several other Cenél nÉogain and Cenél Conaill dynasts. A clear sequence of Uí Néill rulers is not presented, however, by the Old Irish ‘Baile Chuinn’, which also seems to admit Dál nAraide claims. Diarmait's son Áed Sláine was certainly a major force in politics in the period c.598–604 and may well have held the kingship of Tara; there is more compelling evidence that his grandsons Blathmac (qv) and Diarmait Ruanaid (qv), both of whom died in 665, reigned as kings of Tara in the mid seventh century. The descendants of his sons Áed Sláine and Colmán, known as Síl nÁedo Sláine and Clann Cholmáin, emerged as the two principal Uí Néill lineages of the midlands from the seventh century onwards.