Muirchertach/Mac Ercae (d. 536?), son of Muiredach and putatively king of Tara, belonged to the Cenél nÉogain dynasty of the Uí Néill, and is credited with having established its fortunes. According to the pre-Norman genealogies, his father Muiredach was a son of the eponymous Éogan (qv), while the ‘Banshenchas’ (lore of women) names his mother, rather implausibly, as Erc daughter of Loarn, a king of the Scots. He is assigned three brothers (or half-brothers), the most prominent being Feradach, ancestor of Cenél Feradaig.
It is probable that Muirchertach/Mac Ercae was a compound character, created by the fusion of two personal names, the individuals concerned possibly being brothers. The historicity of Mac Ercae is better attested; Adomnán (qv), biographer of Colum Cille (qv), alludes to a Cenél nÉogain uncle and nephew, Báetán (qv) and Eochaid (both slain by the Cianachta in 572), describing the former as a son of Mac Ercae. Yet, it appears that by the Middle Irish period, if not earlier, ‘Mac Ercae’ as a cognomen in its own right was no longer understood. Efforts were made to interpret it as a patronymic, or even a matronymic – as genealogical tradition clearly did not identify his father as Erc, it was postulated that his mother was so named. Mac Ercae's wife is named as Duisech, daughter of Duí Tenga Umae of Uí Briúin of Connacht, while later tradition assigns him several lovers. He is said to have had five sons; in addition to the above-mentioned Báetán, three others are named as Forggus, Domnall, and Néillíne. The fifth son is not identified.
Even accepting that Mac Ercae was an early Cenél nÉogain dynast, a generation before Colum Cille, the reconstruction of his career remains problematic. A non-contemporary stratum of annals associates Muirchertach/Mac Ercae with several sequences of battles, mostly against the Laigin, the dating of which (admittedly somewhat arbitrary) implies an active career of over half a century. On closer examination, however, it is apparent that traditions relating to several different individuals have been amalgamated. Battles fought at Granard, Co. Longford, and Ochae (probably in Mide) dated to the 480s, tentatively ascribed to Mac Ercae, are otherwise credited to Cairpre (qv) son of Niall Noígiallach (qv). Given the early date and the geographical locations, the involvement of Cenél Cairpri rather than Cenél nÉogain seems more plausible.
Aside from fusion with Muirchertach, it appears that Mac Ercae was further confounded with a like-named son of Ailill Molt (qv), an Uí Briúin king. The battle of Segais against Uí Fhiachrach of Connacht is, on balance, more likely to have been fought by Mac Ercae of Uí Briúin – to whom, perhaps, certain encounters with the Laigin in Mide (Co. Westmeath) should also be attributed. The defeat and death of Mac Ercae (to be identified with this individual?) at Tortu (near Ardbraccan, Co. Meath), dated to 543, is expressly credited to the Laigin. The ascription to Muirchertach/Mac Ercae (or at least to his dynasty of Cenél nÉogain) of a further sequence of engagements, clustering in Brega (east Co. Meath), may well be genuine. Included here are the battle of Detnae in Dromma Breg (dated to 520) fought against Colum Cille's dynasty of Cenél Conaill and several encounters with the Laigin: Áth Sige (Assey, Co. Meath) dated to 528 or 530, and Éblenn, Mag nAilbe, and Almu (in a line from east Meath to north Kildare), grouped together at 533 in the Annals of Ulster.
Even if the military role ascribed to Muirchertach/ Mac Ercae is in part acceptable, claims that he held the kingship of Tara deserve further scrutiny. The late seventh-century ‘Baile Chuinn’, which purports to list the kings of Tara, includes a certain Mac Ercéni – whom a gloss in one text of the poem equates with Muirchertach. His placement in the sequence implies that c.507 he succeeded Lugaid son of Lóegaire (qv). However, there are grounds for considering that this early regnal list preserved names of kings of Tara from other dynasties, who were afterwards reinvented as members of Uí Néill – which doubtless explains why Muirchertach/Mac Ercae is expressly featured in the later ‘Baile in Scáil’ and in Middle Irish regnal lists. It is quite possible that the individual intended by the original compilers of ‘Baile Chuinn’ was Mac Ercae son of Ailill Molt, but with the emergence of Cenél nÉogain to a dominant position within Uí Néill in the early eighth century, particularly in the person of Fergal (qv) son of Máel-dúin, it became expedient to back-project the achievement of the dynasty – preferably to a time predating the emergence of their Cenél Conaill and Síl nÁedo Sláine rivals. Muirchertach/Mac Ercae is probably to be identified with the Mac Erca mentioned in ‘Arsiasar coimdhi Temrae’ (known as ‘the Airgialla poem’), seemingly composed in the mid eighth century with the aim of redefining the relationship of the Airgialla kings with Uí Néill in the context of the emergence of Clann Cholmáin; in that event, he was presumably chosen to represent the claims of Cenél nÉogain (in alternation with Cenél Conaill) on the kingship of Tara in the earlier decades of the century.
The death of Muirchertach/Mac Ercae is placed in the annals at 534/6. Some versions of his obit give a curious account of his having drowned in a vat of wine in a house at Cleitech, above the River Boyne. This is taken from the widely discussed Middle Irish death tale ‘Aided Muirchertaig meic Erca’, in which a woman of the faery condemns him to a ‘threefold death’ by wounding, burning, and drowning. So, having been stabbed by assailants, and the house burned around him, he plunges into a vat of wine and perishes. His sons Domnall (d. 566), Forggus (d. 566?), and Báetán (slain 572), and his grandson Eochaid (slain 572) son of Domnall, were presumably prominent dynasts of Uí Néill, but the assertions of Middle Irish sources that all four held the kingship of Tara may be doubted. Of his other grandsons, Fergus son of Néillíne was slain in 570 (or 577) by persons unknown; Colcu son of Domnall was killed in 580 by his Cenél Conaill rival Áed (qv) son of Ainmire (qv); while claims that Colcu's brother Áed Uaridnach (qv) reigned as king of Tara are worthy of consideration, especially as he was ancestor of many later rulers of Cenél nÉogain and of candidates for higher dignities.