Shadowy 5th-century king (c.445) of Connacht, who may or may not have been the last pagan high king of Ireland; sometimes bears the patronymic mac Fiachrach. According to some annals, Dathí was the nephew of Niall Noígiallach [of the Nine Hostages] and king of Connacht before he succeeded his uncle for a twenty-three-year reign. T. F. O'Rahilly believes that Dathí was only a raider to Britain whose story was concocted and interposed by later annalists. According to those stories, Dathí invaded first Strathclyde and later Continental Europe, crossing the Alps; he was struck by lightning after occupying the tower of one Formenus, ‘king of Thrace’, who lived in the Alps as a hermit. For more on this element of his narrative, see Samuel Ferguson, ‘On the Legend of Dathi’, Proceedings of the Royal Irish Academy, 2, 2nd ser. (1884), 167–84. A pillar at Cruachain is thought to mark Dathí's grave, and the Bough of Dathí was one of the sacred trees of early Ireland.
The Dathí of tradition may be unrelated to the non-regal figure of the same name in W. B. Yeats's ‘The Blessed’ (1897). This Dathí is a wise or holy man who tells Cumhall that ‘blessedness goes where the wind goes,/ And when it is gone we are dead.’ See also Mary Genevieve Hogan, The Legend of Dathi: An Analogue to the Chronicle Story of Arthur (Washington, DC, 1933).