Eochaid Ailtlethan ("broad blade"), son of Ailill Caisfiaclach, was, according to medieval Irish legends and historical traditions, a High King of Ireland

High King of Ireland

The High Kings of Ireland were sometimes historical and sometimes legendary figures who had, or who are claimed to have had, lordship over the whole of Ireland. Medieval and early modern Irish literature portrays an almost unbroken sequence of High Kings, ruling from Tara over a hierarchy of...

. According to the Lebor Gabála Érenn

Lebor Gabála Érenn

Lebor Gabála Érenn is the Middle Irish title of a loose collection of poems and prose narratives recounting the mythical origins and history of the Irish from the creation of the world down to the Middle Ages...

, he took the throne after overthrowing and killing the previous incumbent, Adamair

Adamair

Adamair , son of Fer Corb, was, according to medieval Irish legends and historical traditions, a High King of Ireland. He came from Munster, killed the previous incumbent, Ailill Caisfhiaclach, and reigned for five years, until he was killed by Eochaid Ailtleathan...

, and ruled for eleven years, until he was killed in battle by Fergus Fortamail

Fergus Fortamail

Fergus Fortamail , son of Bresal Brecc, son of Óengus Gailian, son of Ailill Bracan, son of Labraid Loingsech, was, according to medieval Irish legend and historical tradition, a High King of Ireland...

. The Lebor Gabála Érenn

Lebor Gabála Érenn

Lebor Gabála Érenn is the Middle Irish title of a loose collection of poems and prose narratives recounting the mythical origins and history of the Irish from the creation of the world down to the Middle Ages...

synchronises his reign with that of Ptolemy V Epiphanes

Ptolemy V Epiphanes

Ptolemy V Epiphanes , son of Ptolemy IV Philopator and Arsinoe III of Egypt, was the fifth ruler of the Ptolemaic dynasty. He became ruler at the age of five, and under a series of regents the kingdom was paralyzed.-Regency infighting:Ptolemy Epiphanes was only a small boy when his father, Ptolemy...

 in Egypt (204-181 BC). The chronology of Geoffrey Keating

Geoffrey Keating

Seathrún Céitinn, known in English as Geoffrey Keating, was a 17th century Irish Roman Catholic priest, poet and historian. He was born in County Tipperary c. 1569, and died c. 1644...

's Foras Feasa ar Éirinn dates his reign to 285-274 BC BC, that of the Annals of the Four Masters

Annals of the Four Masters

The Annals of the Kingdom of Ireland or the Annals of the Four Masters are a chronicle of medieval Irish history...

(which gives him a reign of seventeen years) to 414-396 BC BC.