Fiachrach's most famous son was Daithi, who succeeded Niall of the Nine Hostages as High King of Ireland. Daithi was descended from Heremon, one of the early Celtic High Kings of Ireland. Daithi led his victorious armies through Britain and France and helped to cause the fall of the Roman Empire. He was killed by lightning at the foot of the Alps and his body was brought back to Ireland by his troops, and interred at Relig-na-Riogh, or the burying place of the Kings of Ireland, in County Roscommon. A large red pillar was placed over his grave, which, though considerably weathered over the past fifteen hundred years, can still be seen to this day. Daithi's real name was Feredach, but he was called Daithi, meaning swift, because he was so fast putting on his armour. One story in the ancient manuscripts says that he was so skillful in handling his arms and defending himself, that, if attacked by a hundred persons at once, he could ward off every blow by his dexterity. Daithi was ancestor, not only of the O'Shaughnessys, but also of their kinsfolk, the O'Hynes, O'Cahills, O'Clearys, O'Kilkellys and others.240

He married Felia, daughter of Achy and had issue.211

Went with an army into France and marching over the Alps, was there killed by a thunderbolt. Had 24 sons by 3 wives.9

. . . the youngest but most distinguished of whom was Dathy. So remarkable was he as a successful and brave soldier, that he was proclaimed king and successor to his illustrious father Fiachra. In 406, on the death of Niall, he succeeded as supreme king, having placed his brother Awley on the throne of Connaught Dathy was a monarch of vast ambition, and of extraordinary military powers. He was also remarkable for his skill in the science of self-defense.
His high position as supreme King of Ireland did not satisfy his ambition. Like his heroic predecessor Niall, he resolved to lead his victorious troops to other countries. Our annalists refer to him* as **King of Erin, Alba, Britain, and as far as the mountains of the Alps." Though this language may savor of exaggeration, it helps us to judge of this prince's extraordinary military successes. His authority was respected through every province in Ireland.
Perhaps we can refer to no better proof of this than the fact that he exacted and obtained, without opposition, the Borumean tribute on three successive occasions. Unopposed at home, he was able to assert his authority abroad. Indeed, we see him bearing the Irish flag triumphantly over the remote provinces of Gaul. It must have been with strange feelings that the legions of Gaul found themselves compelled to fly before this invincible barbarian from an almost unknown island in the Northern Ocean. Our historians and annalists abound with glowing accounts of his prowess, which impart a poetic interest to his career. The circumstances under which his extraordinary career was cut short at the foot of the Alps, a.d. 420, accord in their character with that marvelous career, though some of the circumstances may, we think, be received with caution. His death is recorded by OFlaherty in the following simple words: "Dathy, the last of the Irish pagan kings, was killed by lightning at the foot of the Alps, after coming off victorious in one hundred and fifty battles, according to history."241