Blathmac (d. 665), son of Áed Sláine (qv) and king of Tara, was an early representative of the Uí Néill dynasty of Síl nÁedo Sláine. His father, who also reigned as king of Tara, was slain in 604. According to the genealogies, Blathmac had eight half-brothers, the most prominent of whom were Congal, Ailill, Dúnchad and Diarmait Ruanaid (qv) (d. 665). He also had a sister Nonnad, who married Colmán son of Áed of Cenél Lóegaire. Blathmac's mother's name is given as Lann (or Flann); the mother of his half-brothers is said to have been Eithne, daughter of Brénainn. The ‘Banshenchas’ (lore of women) has it that, after his father's death, Blathmac took Eithne to wife. Perhaps this is the basis for the verse in the Annals of Ulster (s.a. 651) which claims that the premature death of two of his sons represented divine retribution for his having ‘begotten a son through excess’. In any event, he would appear to have had at least five sons: Sechnassach, Cenn-fáelad (qv), Dúnchad, Congal, and Eochaid.
It is difficult to pin-point the beginning of Blathmac's career. The slaying (634) of his brothers Congal and Ailill, probably at an early age, may well have opened an opportunity for Blathmac. Congal had reigned as king of Brega, and it may have been at this stage that Blathmac succeeded to the kingship. Significantly, he is portrayed as the secular power behind the expulsion of St Carthach (qv), alias Mochuta, from Rathan, an event that the annals place at 636. The insistence of the Life of Carthach (§§53–5, 58) that Blathmac was already king of Tara raises questions about the date assigned to the saint's expulsion, or about the internal chronology of the Life.
It is quite probable that Blathmac achieved some degree of eminence among the Uí Néill after the slaying of his Clann Cholmáin rival Conall Guthbind (635); it seems unlikely, however, that he should have attained the paramount kingship at this early stage. Ultimately, he did achieve this dignity; the Old Irish regnal poem ‘Baile Chuinn’ includes Blathmac – and also his brother Diarmait Ruanaid – among the kings of Tara. Later sources, including Middle Irish king lists and the Life (§12) of St Gerald (qv) of Mayo, present the two as joint rulers, but the extent to which such alleged joint reigns represent historical reality might be questioned. In this instance, it may be significant that the earlier Life of Carthach maintains that Blathmac was succeeded as king by Diarmait Ruanaid. There are, admittedly, indications that Blathmac derived some benefit from Diarmait's exertions in dealing with opposition from other Uí Néill dynasties. The death (642) of Domnall (qv) son of Áed (qv) of Cenél Conaill, a powerful king of Tara, introduced a period of confusion among the Uí Néill. Later claims that the sons of Máel-Cobo, Conall Cóel (qv) (d. 654) and Cellach (qv) (d. 658), reigned as kings of Tara till 658 are not entirely convincing.
In any case, it appears that Blathmac and his family extended the power of Síl nÁedo Sláine during these years and consolidated the position of their dynasty in the midlands. Presumably, it was in pursuit of such expansionist claims that two of his sons, Dúnchad and Conall, were slain in the Westmeath lakelands by a Leinster dynast, Máelodrán (651). Another son, Eochaid, died violently in 660. The battle of Ogoman (662) would appear to have been an internal conflict within Síl nÁedo Sláine. Blathmac was defeated and many of his supporters slain, including his nephew Conaing son of Congal. Significantly, some of the annalistic accounts claim that the victors were allies of Diarmait Ruanaid. If that is so, the battle may mark the high point of a conflict in which Blathmac was displaced by his brother. As it happens, neither of them survived for long; according to the annals, the two brothers died in the Buide Conaill, the great plague of 665. The kingship passed in turn to two of Blathmac's surviving sons, Sechnassach (d. 671) and Cenn-fáelad (d. 675), neither of whom left descendants of any great importance.