Cairbre Nia Fer son of Rus Ruad, was, according to medieval Irish legend and historical tradition, a King of Tara from the Laigin

Laigin

The Laigin, modern spelling Laighin , were a population group of early Ireland who gave their name to the province of Leinster...

.

The earliest reference to Cairbre is in Tírechán

Tírechán

Tírechán was a 7th century Irish bishop and biographer of Saint Patrick. Tírechán wrote his untitled memoir sometime after the death of his mentor, Ultan of Ardbraccan, in 657. The work survives in the manuscript The Book of Armagh.Tírechán's account, which J. B...

's Memoir of St. Patrick, a 7th century Latin text found in the Book of Armagh

Book of Armagh

The Book of Armagh or Codex Ardmachanus , also known as the Canon of Patrick and the Liber Armachanus, is a 9th-century Irish manuscript written mainly in Latin. It is held by the Library of Trinity College, Dublin...

. Patrick finds an enormous grave and raises its giant occupant from the dead. The giant says he was killed by the sons of Mac Con

Lugaid mac Con

Lugaid Mac Con, often known simply as Mac Con, was, according to medieval Irish legend and historical tradition, a High King of Ireland. He belonged to the Corcu Loígde, and thus to the Dáirine. His father was Macnia mac Lugdach, and his mother was Sadb ingen Chuinn, daughter of the former High...

 during the reign of Cairbre Nia Fer, a hundred years previously – i.e. in the 4th century.

The 11th century 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...

places him during the reign of the High King Eterscél, which it synchronises his reign with that of the Roman emperor

Roman Emperor

The Roman emperor was the ruler of the Roman State during the imperial period . The Romans had no single term for the office although at any given time, a given title was associated with the emperor...

 Augustus

Augustus

Augustus ;23 September 63 BC – 19 August AD 14) is considered the first emperor of the Roman Empire, which he ruled alone from 27 BC until his death in 14 AD.The dates of his rule are contemporary dates; Augustus lived under two calendars, the Roman Republican until 45 BC, and the Julian...

 (27 BC - AD 14) and the birth of Christ

Christ

Christ is the English term for the Greek meaning "the anointed one". It is a translation of the Hebrew , usually transliterated into English as Messiah or Mashiach...

, and makes him a contemporary of the provincial kings 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:...

 of the Ulaid

Ulaid

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

, Cú Roí

Cú Roí

Cú Roí mac Dáire is a king of Munster in the Ulster Cycle of Irish mythology. He is usually portrayed as a warrior with superhuman abilities and a master of disguise possessed of magical powers. His name probably means "hound of the plain/field", or more specifically, "hound of the battlefield"...

 of Munster

Munster

Munster is one of the Provinces of Ireland situated in the south 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 administrative and judicial purposes...

 and Ailill mac Máta

Ailill mac Máta

Ailill mac Máta is the king of the Connachta and the husband of queen Medb in the Ulster Cycle of Irish mythology. He rules from Cruachan .-Family background, marriage and offspring:...

 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...

. Mac Con of the Dáirine

Dáirine

The Dáirine , later known dynastically as the Corcu Loígde, were the proto-historical rulers of Munster before the rise of the Eóganachta in the 7th century AD. They appear to have derived from the Darini of Ptolemy and to have been related to the Ulaid and Dál Riata of Ulster and Scotland...

, placed a generation before Cairbre by Tírechán, is dated many generations after him, to the late 2nd century, in the Lebor Gabála.

Alongside Conchobar, Cú Roí and Ailill, Cairbre appears as king of Tara in 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...

, where he is the brother of Ailill mac Máta

Ailill mac Máta

Ailill mac Máta is the king of the Connachta and the husband of queen Medb in the Ulster Cycle of Irish mythology. He rules from Cruachan .-Family background, marriage and offspring:...

, husband of Medb

Medb

Medb – Middle Irish: Meḋḃ, Meaḋḃ; early modern Irish: Meadhbh ; reformed modern Irish Méabh, Medbh; sometimes Anglicised Maeve, Maev or Maive – is queen of Connacht in the Ulster Cycle of Irish mythology...

 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...

. His wife is Fedelm Noíchrothach

Fedelm Noíchrothach

Fedelm Noíchrothach , also known as Fedelm Noíchride , is a daughter of Conchobar mac Nessa in the Ulster Cycle of Irish mythology....

, daughter of Conchobar, and they have a son, Erc

Erc mac Cairpri

Erc mac Cairpri is a character from the Ulster Cycle of Irish mythology. His is the son of Cairpre Nia Fer, king of Tara, and his wife Fedelm Noíchrothach, daughter of Conchobar mac Nessa, king of Ulster...

, and a daughter, Achall

Achall

Achall, daughter of Cairbre Nia Fer, king of Tara, and his wife Fedelm Noíchrothach, is a minor character from the Ulster Cycle of Irish mythology...

. In Cath Ruis na Ríg ("The Battle of Rosnaree

Rosnaree

Rossnaree is a small village in County Meath, Ireland, on the south bank of the River Boyne, near the Brú na Bóinne complex of neolithic monuments on the north bank. It commands a ford that was used by the Williamites at the Battle of the Boyne in 1690...

"), he and his brother Find mac Rossa, king of the Gailióin of Leinster

Leinster

Leinster is one of the Provinces of Ireland situated in the east of Ireland. It comprises the ancient Kingdoms of Mide, Osraige and Leinster. Following the Norman invasion of Ireland, the historic fifths of Leinster and Mide gradually merged, mainly due to the impact of the Pale, which straddled...

, fight a battle against Conchobar and the Ulaid. The Ulaid hero Cúchulainn

Cúchulainn

Cú Chulainn or Cúchulainn , and sometimes known in English as Cuhullin , is an Irish mythological hero who appears in the stories of the Ulster Cycle, as well as in Scottish and Manx folklore...

 kills him with a spear from distance, then decapitates him before his body hits the ground. After the Ulaid win the battle, Cairbre's son Erc swears allegiance to Conchobar, marries Cúchulainn's daughter Finnscoth, and becomes king of Tara in his father's place.

Erc is later part of the conspiracy to kill Cúchulainn. After avenging Cúchulainn, Conall Cernach

Conall Cernach

Conall Cernach is a hero of the Ulaidh in the Ulster Cycle of Irish mythology. He is said to have always slept with the head of a Connachtman under his knee. His epithet is normally translated as "victorious" or "triumphant", although it is an obscure word, and some texts struggle to explain it...

brings Erc's head back to Tara, where Achall dies of grief for her brother.