Niall Frossach (718–78), son of Fergal and king of Tara, belonged to the Uí Néill dynasty of Cenél nÉogain. His father, Fergal (qv) son of Máel-dúin, was slain in 722 and his half-brother Áed Allán (qv) in 743. Both preceded Niall in the kingship of Tara. He had two other half-brothers, Conchobar and Colcu. His mother was Aithechda, daughter of Cian, whose lineage is not recorded. According to a retrospective entry in the Annals of Ulster, Niall was born in 718. He was married to Dúnlaith (d. 799), daughter of the Cenél Conaill king Flaithbertach (qv) son of Loingsech (qv), who was the mother of his son Áed Oirdnide (qv). He also had four other sons by some unrecorded mother (or mothers): Ferchar, Muirchertach, Gormgal (d. 814), and Colmán (slain 815).There is no evidence to show that he was ever married to Eithne daughter of Bressal Breg, who is styled ‘queen of the kings of Tara’ in her obit at 768.
A range of sources purport to explain Niall's sobriquet Frossach (showery), with claims that at his birth showers of wheat and/or silver fell on the fortifications of the Uí Néill, while a shower of honey (or blood) fell on the ramparts of the Laigin. Such traditions probably reflect memories of his reign as a time of prosperity, or of recovery from famine. Other stories represent Niall as peaceful in character and with religious inclinations, in sharp contrast to his warlike brother Áed Allán. He is also the subject of a poem by Gilla Mo-Dutu Ua Casaide (qv), preserved in the Book of Ballymote.
Niall may well have controlled Cenél nÉogain from 743, when Áed Allán was slain. His dominance of the Uí Néill of the north was recognised four years later, following the death of the Cenél Conaill dynast Áed Muinderg son of Flaithbertach. It seems that Niall posed a threat to the powerful Clann Cholmáin king of Tara, Domnall Midi (qv); in 756 the latter led a hosting against him to Mag Muirtheimne in Co. Louth, but without conclusive results. For his part, Niall was not in a position to assert his claims to the kingship of Tara till 763, when Domnall died. Although he achieved no significant military victories, Niall did exact tributes from Connacht, Munster, and Leinster. By the end of the decade, however, his position was being undermined with the rise of Donnchad Midi (qv) son of Domnall. Whether Niall's reign came to an effective end in 770 (the regnal lists credit him with only seven years as king of Tara) or two years later, when Donnchad hosted into Tír nÉogain, is not clear, but he was forced to abdicate and died in religious retirement on Iona in 778.
It appears that Cenél nÉogain fortunes declined in the later years of Niall's reign, facilitating a temporary revival of Cenél Conaill power. His nephew Máel-dúin, who had followed him as ruler of Cenél nÉogain, eventually managed to assert his claims as king of Ailech; in 788 Niall's son Áed Oirdnide succeeded to that dignity and later became king of Tara; his other sons, including Gormgal (d. 814), were ancestors of relatively unimportant lines.