Muirchertach, according to genealogies belonged to the Uí Néill and was the son of Muiredach, son of Eógan, son of Niall of the Nine Hostages, hence Muirchertach mac Muiredaig. His mother, "clearly legendary" according to Charles-Edwards, was said to be Erc, daughter of "Lodarn, king of Alba". From the matronym comes his alternative name, Muirechertach Macc Ercae. However, Mac Ercae was a common enough first male first name. The annalistic entries for Muirchertach span fifty years, from 482 to his death in 534, using various names, including Mac Ercae, so that it is more than likely that two or more persons have been confused in the annals.
The first mentions of Muirchertach in the Annals of Ulster, in 482 and 483, associate him, under the name Muirchertach Macc Ercae, with the defeat and killing of Ailill Molt at the battle of Ochae, somewhere in the Irish midlands. One entry names Lugaid mac Lóegairi as his ally there, the other names Fergus Crook-mouth, father of Diarmait mac Cerbaill. In 485 are mentioned the battle of Grainert, perhaps near Castledermot, where Coirpre mac Néill, "or Mac Ercae...as other state", defeated the Leinster king Finnchad mac Garrchon. In 490 or 491, Óengus mac Nad Froích is said to have been killed at the battle of Cell Losnaid, and the second entry reports that "Mac Ercae was the victor". Muirchertach Mac Ercae is said to have won the battle of Inne Mór against the Leinstermen in 498. Many of the entries from the 480s and 490s appear to have been modified, to give Muirchertach the credit for victories won by Coirpre mac Néill and perhaps by Coirpre's son Eochu.
The obituary of Lugaid mac Lóegairi appears in 512, and in the following year the annalist reports the beginning of the reign of Muirchertach Macc Ercae. The next report is in 520, duplicated in 523, stating that Muirchertach was among the victors at the battle of Dethna. Another battle follow in 528, again repeated some years later, in 533, with more detail.
Muirchertach's death is reported in 534, with obvious supernatural overtones:
The drowning of Muirchertach Mac Erca i.e. Muirchertach son of Muiredach son of Eógan son of Niall Naígiallach in a vat full of wine on the hilltop of Cleitech above Bóinn.
Muirchertach was said to be followed as High King by Túathal Máelgarb. Muirchertach's descendants in time took the name Cenél maic Ercae and were the dominant branch of the Cenél nEógan by the middle of the 8th century.
The 12th century Middle Irish language tale Aided Muirchertaig Meic Erca is an account of the supernatural death of Muirchertach. Here Muirchertach dies in the House of Clettach, drowned in a vat of wine, burned by fire, and crushed by a falling roof beam, near Brú na Bóinne, beguiled by the illusions of the otherworldly maiden Sín into believing that he is being attacked by Túathal Máelgarb.