[6] However, the early annals record the activities of his sons between 429 and 516, an implausibly long period for a single generation, leading scholars like Kathleen Hughes[4] and Francis J. Byrne[3]: 78–79  to conclude that the events of the later half of the 5th century have been extended backwards to accommodate as early a date as possible for the arrival of Saint Patrick, with the effect of pushing Niall back up to half a century.

The High Kingship did not become a reality until the 9th century, and Niall's status has probably been inflated in line with the political importance of the dynasty he founded.

[7]: 222–232  O'Rahilly and Byrne argue that the literary sources, though late and garbled, preserve genuine traditions that Niall led raids on Britain, and perhaps died on one.

[3]: 76–78 [7]: 220 Professor Dáithí Ó hÓgáin seems to indicate that Niall was probably a descendant of the Gaulish seafaring tribe of the Veneti, who originated in south-east Armorica (modern Brittany, France).

The southern Venii came to be known as the Eoghanacht, while another group of them migrated north and formed a new kingdom west of the River Shannon where they became known as the Connachta after a revered leader named Conn.

These Connachta later extended their power eastward into the plain of Meath, and under the leader Teutovalos Teachtmhar overthrew the kingship of the Lagini at Tara around AD 300.

The Primitive Irish Vendo is a cognate with Finn, and the Fianna were landless, aristocratic young men and women who had not yet come into their inheritance of land.

[8] A biography of Niall can be constructed from sources such as the "Roll of Kings" section of the 11th-century Lebor Gabála Érenn, the Annals of the Four Masters, compiled in the 17th-century, chronicles such as Geoffrey Keating's Foras Feasa ar Éirinn (1634), and legendary tales like the 11th-century "The Adventure of the Sons of Eochaid Mugmedon" and "The Death of Niall of the Nine Hostages".

A legendary account of Niall's birth and early life is given in the possibly 11th-century tale Echtra mac nEchach Muimedóin ("The adventure of the sons of Eochaid Mugmedón").

While Cairenn is pregnant with Niall, the jealous Mongfind forces her to do heavy work, hoping to make her miscarry.

Niall, who emerges carrying an anvil, is deemed greater than Brión, with a sledgehammer, Fiachrae with bellows and a pail of beer, Ailill with a chest of weapons, and Fergus with a bundle of wood.

Variations of this story are told of the earlier Irish high king Lugaid Loígde, in Arthurian legend—one of the most famous versions appears in both Geoffrey Chaucer's The Wife of Bath's Tale and the related Gawain romance, The Wedding of Sir Gawain and Dame Ragnelle—and in John Gower's Middle English poem Confessio Amantis.

Mongfind, purporting to make peace between her brother and her sons, holds a feast, at which she serves Crimthann a poisoned drink.

The Munstermen renew the battle, capture Ailill and cut him to pieces, and war continues between Munster and Connacht for many years.

Abruptly, the tale then has Niall appearing before an assembly of Pictish bards in Scotland, where he is killed by an arrow shot by Eochaid from the other side of the valley.

[13] He is succeeded by his nephew Nath Í. Byrne suggests that Niall's death took place during a raid on Roman Britain.

For instance, the chiefs of Clan Donald are now known to belong to a branch of Haplogroup R1a, which split from Niall's hypothetical lineage over 20,000 years ago.

[19] Following a 2006 hypothesis by Moore et al.[20] suggesting that his Y chromosomal signature had been discovered, popular science journalists and genetic testing companies began promoting the theory that millions alive today have an unbroken descent from Niall.

Moore et al. concluded that these men descend from "a single early-medieval progenitor" and implied this was associated with Niall's dynasty.

Furthermore, the paper examined only 17 STR loci, which are not a reliable means of verifying descent, as SNPs, which define haplogroups and subclades, would be.

[6] O'Rahilly suggests that the nine hostages were from the kingdom of the Airgialla (literally "hostage-givers"), a satellite state founded by the Ui Néill's conquests in Ulster, noting that the early Irish legal text Lebor na gCeart ("The Book of Rights") says that the only duty of the Airgialla to the King of Ireland was to give him nine hostages.