According to the 12th century Lebor Gabála Érenn, he took power when he defeated the previous High King, Fachtna Fáthach, in the Battle of Leitir Rúaid. The Middle Irish saga Cath Leitrech Ruibhe tells the story of this battle. While Fachtna Fáthach was away from Tara on a visit to Ulster, Eochu, then king of Connacht, raised an army, had the provincial kings killed and took hostages from Tara. When news reached Fachtna at Emain Macha, he raised an army of Ulstermen and gave battle at Leitir Rúaid in the Corann (modern County Sligo), but was defeated and beheaded by Eochu. Eochaid Sálbuide, the king of Ulster, was also killed. Fergus mac Róich covered the Ulster army's retreat, and Eochu marched to Tara.
Various Middle Irish tales give him a large family. His wife was Cloithfinn, and they had six daughters, Derbriu, Eile, Mugain, Eithne, Clothru and Medb, and four sons, a set of triplets known as the three findemna, and Conall Anglondach. Derbriu was the lover of Aengus of the Tuatha Dé Danann. Her mother-in-law, Garbdalb, turned six men into pigs for the crime of eating nuts from her grove, and Derbriu protected them for a year until they were killed by Medb. When Conchobar mac Nessa became king of Ulster, Eochu gave four of his daughters, Mugain, Eithne, Clothru and Medb, to him in marriage in compensation for the death of his supposed father, Fachtna Fáthach. Eithne bore him a son, Furbaide, who was born by posthumous caesarian section after Medb drowned her. Clothru, according to one tradition, bore him his eldest son Cormac Cond Longas, although other traditions make him the son of Conchobar by his own mother, Ness. Medb bore Conchobar a son called Amalgad, but later left him, and Eochu set her up as queen of Connacht. Some time after that, Eochu held an assembly at Tara, which both Conchobar and Medb attended. The morning after the assembly, Conchobar followed Medb down to the river Boyne where she had gone to bathe, and raped her. Eochu made war against Conchobar on the Boyne, but was defeated.
The three findemna tried to overthrow their father in the Battle of Druimm Criaich. The night before the battle, their sister Clothru, afraid that they would die without an heir, seduced all three of them, and the future High King Lugaid Riab nDerg, was conceived. The next day they were killed, and their father, seeing their severed heads, swore that no son should directly succeed his father to the High Kingship of Ireland.
He ruled for twelve years, and died of natural causes at Tara, succeeded by his brother Eochu Airem. The Lebor Gabála synchronises his reign with the dictatorship of Julius Caesar (48-44 BC). The chronology of Geoffrey Keating's Foras Feasa ar Éireann dates his reign to 94-82 BC, that of the Annals of the Four Masters to 143-131 BC.