Áed Sláine (d. 604), son of Diarmait and king of Tara, belonged to the Uí Néill dynastic confederation and was ancestor of the Síl nÁedo Sláine (‘the seed of Áed of Slane’) dynasty of Brega. It is probably significant that the leading dynasties of what became the Southern Uí Néill – Sil nÁedo Sláine and Clann Cholmáin – derive their origin from Diarmait (qv) (d. 565) son of Cerball, rather than from Niall Noígiallach (qv) directly, Diarmait purportedly being the latter's great grandson. Middle Irish genealogists represent Áed and his alleged half-brothers, Colmán Már (qv) and Colmán Bec (qv), ancestors respectively of Clann Cholmáin and Caílle Follamain, as sons of Diarmait (king of Tara 544–65).
Áed Sláine's mother, according to tradition, was Mugain daughter of Cúchraide son of Duí of the Uí Duach, a dynasty later subsumed into the Éoganachta. An episode in the Latin Life of Áed son of Brecc (§14) has the saintly abbot cure her of sterility. The striking birth tale ‘Genemain Áeda Sláine’ (in a tenth-century text) relates that his mother gave birth first to a lamb, secondly to a silver fish, and thirdly to Áed Sláine. Áed is said to have married Eithne (probably, in fact, a compound character), daughter of Brénainn Dall of the Conmaicne Cúile Tolad; she is claimed as the mother of up to seven of Áed's sons, including Diarmait Ruanaid (qv), Congal, Ailill, and Dúnchad. Another wife, a certain Lann, was allegedly the mother of his son Blathmac (qv). He also had a daughter Nonnad, who married Colmán son of Áed of the Cenél Lóeguire.
Áed Sláine, who emerged in the closing years of the sixth century, should perhaps be identified with ‘Aíd’ of the Old Irish regnal poem ‘Baile Chuinn’, which would mean that he attained the kingship of Tara; later king-lists claim that Colmán Rímid (qv) of the Cenél nÉogain reigned jointly with him. According to the hagiographers, Áed was associated with St Colmán (qv) of Lann Elo. He is said to have donated the site of the monastery (Lynally, Co Offaly) to the saint and, later, to have released a prisoner at the holy man's request (‘Vita S. Colmani’, §§3, 24). He is featured in a less positive light in Columban hagiography. Adomnán (qv), the late seventh-century abbot of Iona and biographer of Colum Cille (qv), has the saint warn Áed to avoid fingal, the slaying of kindred (‘Vita Columbae’, i, §14). He cautions: ‘My son, you must take heed lest by reason of the sin of parricide [sic for fratricide] you lose the prerogative of monarchy over the kingdom of all Ireland, predestined for you by God’ (tibi a deo totius Everniae regni praerogativam monarchiae praedestinatam). This (presumably post factum) prophecy appears to have been fulfilled when in 600, as the record shows, Áed slew his nephew and rival Suibne son of Colmán (probably to be equated with Colmán Bec), an act that caused his downfall and the partition of the Uí Néill overkingship.
In retaliation, in 604 Áed was assassinated on the orders of Suibne's son Conall Guthbind, the actual killing being performed by one of Conall's foster-brothers, Áed Guastán. Historically, there are grounds for considering that Áed's victim Suibne, whose father Colmán was active in Dál Riata and who was quite possibly named after Colum Cille, was closer to Columban and to Cenél Conaill interests than was later acknowledged. Such interests were hardly grateful to Áed for his part in the internecine strife that facilitated the emergence of Cenél nÉogain as strong contenders for Uí Néill overkingship in the course of the seventh century, and which drove deep divisions between his own descendants and those of Colmán. Áed's immediate successors in the kingships of Brega and Tara were respectively Fergus son of Colmán and Áed Uaridnach (qv) of Cenél nÉogain. His son Congal ruled Brega for some years up to 634, when he was slain by Clann Cholmáin. Two of his younger sons, Blathmac and Diarmait Ruanaid, later held the kingship of Tara.