Fachtna Fáthach son of Cas (or Ross), son of Rudraige

Rudraige mac Sithrigi

Rudraige, son of Sitric, son of Dub, son of Fomor, son of Airgetmar, was, according to medieval Irish legend and historical tradition, a High King of Ireland. He took power after killing his predecessor, Crimthann Coscrach, and ruled for thirty or seventy years, after which he died of plague in...

, was, according to medieval Irish legend and historical tradition, 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...

. He came to power when he defeated the previous High King, Dui Dallta Dedad, in the battle of Árd Brestine. According to some stories he was the lover of Ness

Ness (Irish mythology)

Ness , also called Nessa, is a princess of the Ulaid and the mother of Conchobar mac Nessa in the Ulster Cycle of Irish mythology. Her father is Eochaid Sálbuide, king of the Ulaid....

, daughter of Eochaid Sálbuide

Eochaid Sálbuide

Eochaid Sálbuide was king of Ulster prior to the events of the Ulster Cycle of Irish mythology. His daughter was Ness. He died at the Battle of Leitir Ruadh, fighting on the side of the High King of Ireland, Fachtna Fáthach, against Eochaid Feidlech, who defeated and deposed him as High King. He...

, king of Ulster

Ulaid

The Ulaid or Ulaidh were a people of early Ireland who gave their name to the modern province of Ulster...

, and the father of her son, Conchobar mac Nessa

Conchobar mac Nessa

Conchobar mac Nessa was the king of Ulster in the Ulster Cycle of Irish mythology. He ruled from Emain Macha .-Birth:...

, the king of Ulster in the stories of the Ulster Cycle

Ulster Cycle

The Ulster Cycle , formerly known as the Red Branch Cycle, one of the four great cycles of Irish mythology, is a body of medieval Irish heroic legends and sagas of the traditional heroes of the Ulaid in what is now eastern Ulster and northern Leinster, particularly counties Armagh, Down and...

.

After he had reigned for sixteen or twenty-five years, he paid a visit to Ulster. While he was there, Eochu Feidlech, king of Connacht

Connacht

Connacht , formerly anglicised as Connaught, is one of the Provinces of Ireland situated in the west of Ireland. In Ancient Ireland, it was one of the fifths ruled by a "king of over-kings" . Following the Norman invasion of Ireland, the ancient kingdoms were shired into a number of counties for...

, raised an army and marched on Tara

Hill of Tara

The Hill of Tara , located near the River Boyne, is an archaeological complex that runs between Navan and Dunshaughlin in County Meath, Leinster, Ireland...

. With the support of the Ulstermen, Fachtna challenged him to battle. Eochu agreed, and named the battlefield as Leitir Ruad in the Corann

Corann

The Corann was a barony in County Sligo in the province of Connacht, Ireland....

, Connacht. During the battle Eochu surrounded and beheaded Fachtna, and became High King in his place.

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 Fachtna's reign with the Roman civil war

Caesar's civil war

The Great Roman Civil War , also known as Caesar's Civil War, was one of the last politico-military conflicts in the Roman Republic before the establishment of the Roman Empire...

 between Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

 and Pompey

Pompey

Gnaeus Pompeius Magnus, also known as Pompey or Pompey the Great , was a military and political leader of the late Roman Republic...

 (49 BC) and the reign of Cleopatra (51-30 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 Éireann dates his reign to 110-94 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...

to 159-143 BC;