thumb|250px|right|"Death of King Dahi", illustration by [[John Fergus O'Hea from A. M. Sullivan, The Story of Ireland, 1867]]
Nath Í, also known as Dathí, son of Fiachrae, son of Eochaid Mugmedon, was a semi-historical Irish king of the 5th century, the father of the likely-historical king Ailill Molt and the ancestor of the Uí Fiachrach dynasties of early medieval Connacht. His mother was Béḃinn. According to legend, he was a High King of Ireland, and died after being struck by lightning while on an expedition to the Alps.
History
Historically, Nath Í is primarily known for his descendants. His son Ailill Molt was likely a historical 5th century king. Two more sons, Echu and Fiachnae, were the ancestors of the Uí Fiachrach Aidni and the Uí Fiachrach Muaide respectively, both early medieval dynasties in Connacht. A fourth son, Amalgaid, was the ancestor of Tírechán, the 7th century bishop and biographer of St. Patrick. the Annals of the Four Masters and Geoffrey Keating's Foras Feasa ar Éirinn, place him in the traditional list of High Kings of Ireland, after Niall of the Nine Hostages and before Niall's son Lóegaire, with Nath Í's son, Ailill Molt, succeeding Lóegaire. However, he is not included in the earliest list of kings of Tara, the Baile Chuinn Chétchathaig. However, Nath Í's name does not appear on the Connacht regnal lists, and Ailill Molt's does.
An early list of Nath Í's battles takes him outside Ireland, including battles in Strathclyde and Kincardine in Scotland, and an expedition across the English Channel to the Alps.
O'Rahilly and Francis J. Byrne observe out that Nath Í's death-tale is modelled on that of Niall of the Nine Hostages, and Keating
A fuller version of his death-tale, as mentioned to in the annals, is found in the saga Aided Nath Í ("the death of Nath Í"), a later version of the Lebor Gabála, and Keating, in which he besieges a tower in which Forménus, king of Thrace, lives as a hermit, having forsaken his kingdom for a religious life. Forménus prays that God will punish him, and he is struck by lightning, although the Lebor Gabála adds that "scholars suppose" Forménus shot him with an arrow.