Diarmait Ruanaid (d. 665), son of Áed Sláine and king of Tara, belonged to the first generation of the Uí Néill dynasty of Síl nÁedo Sláine. His father, Áed Sláine (qv), whose death is placed at 604, reigned as king of Tara. His mother is named as Eithne, daughter of Brénainn, and the genealogists accord him six brothers or half-brothers, the most distinguished of whom is Blathmac (qv) (d. 665). It seems clear that by Diarmait's time his dynasty, or at least the branch that settled around Lagore, had fostered political alliances with the neighbouring Cruthin kingdoms, which later became known as the Airgialla. In particular, they had connections with the Mugdorna, whose realm straddled the north Meath–Monaghan border. Diarmait was apparently fostered by Lommainech, king of the Mugdorna; the latter is called aite Diarmata in the Book of Leinster.
According to the ‘Banshenchas’ (lore of women), Diarmait was married to Temair, daughter of Áed Bolg king of the Déisi (quite possibly, the Déisi of Tara is intended) – the mother of Diarmait's only recorded son, Cernach Sotal. Elsewhere, however, Diarmait is said to have had a wife named Mugain. It may be significant that the names of both these wives have clear connotations of sovereignty. A similar theme perhaps underlies the Middle Irish tale that associates Diarmait with a mythical woman named Becfhola, who betrays him. In a rather improbable episode in the Life of Féchín (qv) of Fore (§13), the saint inveigles an unnamed wife of Diarmait into participating in a bizarre miracle of healing involving a leper.
The historical record suggests that Diarmait first came to prominence as he and Blathmac strove to secure the primacy of their dynasty within the Uí Néill political structure when their brothers, Congal and Ailill, rulers of Brega, were killed (634) by Conall Guthbinn of Clann Cholmáin. Diarmait, still a young man, enlisted the support of his Airgialla allies, defeating and slaying Conall Guthbinn the following year. His foster-father Lommainech ensured that Conall's son perished at the battle of Mag Roth (Moira, Co. Down; 637). The confusion that prevailed among the Uí Néill in the north-west after the death (642) of Domnall (qv) son of Áed (qv) of Cenél Conaill, styled rex Hiberniae (king of Ireland), facilitated Diarmait and his surviving brothers in extending their influence in the midlands.
In 649 Diarmait defeated the Connacht king Guaire Aidni (qv) at Carn Conaill (near Gort, Co. Galway), in a battle that was commemorated in a king-tale. Either in this engagement, or in a subsequent encounter in the Westmeath lakelands, Diarmait lost two of his nephews: Dúnchad and Conall, sons of Blathmac. Ultimately, Diarmait and Blathmac did secure the paramount Uí Néill overkingship; the Old Irish regnal poem ‘Baile Chuinn’ lists both brothers as 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, and assign them a reign of ten years. One annalist places the commencement of this joint reign at 656, but it is probable that the Síl nÁedo Sláine ascendancy was established before this, their alleged predecesors Cellach and Conall Cóel (qv), nephews of Domnall son of Áed, being shadowy figures.
In any event, there are indications that Diarmait and his brother may not in fact have shared power amicably. The thirteenth-century Life of St Carthach (qv) suggests that Diarmait opposed Blathmac's moves to expel the saint from Rahan, which is supposed to have gained him the sobriquet Ruanaid (the heroic) and the promise that he would succeed to the kingship. Some annalists credit Diarmait with a key role in an internal Síl nÁedo Sláine conflict which culminated in the defeat of Blathmac at the battle of Ogoman (662). Several of the latter's supporters, including his nephew Conaing son of Congal, were slain in this encounter. A transition of power, with Diarmait gaining the kingship at his brother's expense, may well have occurred at this point. Be that as it may, the brothers did not survive for long afterwards.
According to the annals, Diarmait and Blathmac both perished in the Buide Conaill, as the great plague of 665 was called. In the short term, the kingship was not retained by Diarmait's immediate family, but passed to his nephews Sechnassach and Cenn-fáelad (qv) (d. 675), sons of Blathmac. Diarmait, however, through his son Cernach Sotal (d. 688), became the ancestor of Clann Chernaig Shotail, a royal lineage based at Lagore. The most distinguished ruler of this line was Fogartach (qv) (d. 724) son of Niall.