Niall Noígíallach (pronounced [ˈn̠ʲiːa̯l̪ n̪oːi̯ˈɣʲiːa̯l̪əx]; Old Irish "having nine hostages"), or Niall of the Nine Hostages, was a legendary, and possibly historical, Irish king. He was reputedly a High King of Ireland, and the progenitor of the Uí Néill dynasties who dominated Ireland from the 6th to the 10th centuries. Although Irish annalistic and chronicle sources place his reign in the late 4th and early 5th centuries, modern scholars date him about half a century later.

Niall is presumed, based on the importance of his sons and grandsons, to have been a historical person, but the early Irish annals say little about him. The Annals of Inisfallen date his death before 382, and the Chronicon Scotorum to 411. The later Annals of the Four Masters dates his reign to 379–405, and the chronology of Geoffrey Keating's Foras Feasa ar Éirinn to 368–395.

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 and Francis J. Byrne 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. Hughes says "Niall himself must have died not before the middle of the fifth century". Byrne, following James Carney, is a little more precise, dating his death to c. 452.

Niall is placed in the traditional list of High Kings of Ireland. However, the traditional roll of kings and its chronology is now recognised as artificial. 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.

T. F. O'Rahilly argued that Niall and his sons were responsible for the breakup of the ancient kingdom of Ulster and the creation of the kingdoms of Tír Chonaill and Tír Eoghain, and the satellite kingdom of the Airgíalla. 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.

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"). In it, Eochaid Mugmedón, the High King of Ireland, had five sons: four, Brión, Ailill, Fiachrae and Fergus, by his first wife Mongfind, sister of the king of Munster, Crimthann mac Fidaig; and a fifth, Niall, by his second wife Cairenn Chasdub, daughter of Sachell Balb, king of the Saxons. While Cairenn is pregnant with Niall, the jealous Mongfind forces her to do heavy work, hoping to make her miscarry. She gives birth as she is drawing water, but out of fear of Mongfind, she leaves the child on the ground, exposed to the birds. The baby is rescued and brought up by a poet called Torna. When Niall grows up he returns to Tara and rescues his mother from her labour.

Although it is anachronistic for Niall's mother to have been a Saxon, O'Rahilly argues that the name Cairenn is derived from the Latin name Carina, and that it is plausible that she might have been a Romano-Briton. Keating describes her not as a Saxon but as the "daughter of the king of Britain". Mongfind appears to have been a supernatural personage: the saga "The Death of Crimthann mac Fidaig" says the festival of Samhain was commonly called the "Festival of Mongfind", and prayers were offered to her on Samhain eve.