Diarmait mac Cerbaill (died ) was King of Tara or High King of Ireland. According to traditions, he was the last High King to follow the pagan rituals of inauguration, the ban-feis or marriage to goddess of the land. The last High King to observe the ancient pagan Feis Temrach or Assembly of Tara which took place on Samhain every three years to pass or renew laws, approve annals and records.

While many later stories were attached to Diarmait, he was a historical ruler and his descendants were of great significance in Medieval Ireland.

Sources
It is believed that the earliest of the Irish annals which came to make up the lost Chronicle of Ireland were kept as a contemporary record from no later than the middle of the 7th century, and may be rather older as it has been argued that many late 6th century entries have the appearance of contemporary recording. There is general agreement that the annals are largely based, in their earliest contemporary records, on a chronicle kept at the monastery on Iona, and that the recording moved to somewhere in the midlands of Ireland only around 740. Although it is thus possible that the records of Diarmait's times in the annals are nearly contemporary, the later history of the annals is complex and much debated, so that it is uncertain to what extent surviving late annals such as the Annals of Ulster and Annals of Tigernach faithfully reproduce the earlier records.

Diarmait mac Cerbaill was the last to hold the sacral kingship of Tara.  He has also some title to be ranked as the first Christian high-king of Ireland.  Two of his sons bore the specifically Christian name of Colmán, deriving from the Latin Columbanus.  This ambivalent character, together with the fact that he was the direct ancestor of the two most powerful dynasties of the Southern Uí Néill, made him an obvious figure for saga and legend.

Descendants of Niall
Diarmait was the son of Fergus Cerrbél, son of Conall Cremthainne, son of Niall of the Nine Hostages. Yet of Niall's own historicity there is little reason to doubt.  His descendants quarrelled incessantly among themselves after the manner of most Irish dynastic families and had no cause to invent a common ancestry, since by unanimous testimony the high-kingship of Tara prior to Niall's days had not been the preserve of any one tribe or family.  By the end of the fifth century, however, it was well on the way to becoming so.  Niall's sons and grandsons proclaimed their intention of monopolising it to the exclusion of their western cousins by discarding the tribal appellation of Connachta and adopting the dynastic name Uí Néill, nepotes Néill.  As a great-grandson of Niall, Diarmait and his descendants were counted among the Uí Néill, the name meaning "descendants of Niall".

The two great Southern Uí Néill dynasties of the midlands were the Síl nÁedo Sláine (the Seed of Áedo of Slane), kings of Brega in the east, and the Clann Cholmáin Máir (the Children of Colmán the Great) in Mide with their centre in the heart of modern Westmeath.  The former are more prominent in the seventh century, but after the death of Cináed mac Írgalaig in 728 all the high-kings of the Southern Uí Néill come from the Clann Cholmáin except for a brief period between 944 and 956 when the king of Knowth, Congalach Cnogba, restored the high-kingship to the Brega line.

It is remarkable that the Síl nÁedo Sláine and Clann Cholmáin derive their origin, not directly from Niall Noígiallach, but from his great-grandson Diarmait mac Cerbaill.  The annals date Diarmait's reign as high-king from about 544 to 565.  The petty Uí Néill kings of Cenél nArdgail traced their ancestry to an uncle of Diarmait's, but never won the high kingship.  Besides Colmáin Már and Áed Sláine, Diarmait had a third son Colmáin Bec, whose descendants, the dynasty of Caílle Follamain, ruled an area corresponding to the baronies of Fore, between Mide and Brega.

Diarmait's immediate origins are obscure and may arouse some suspicion.  In spite of his patronymic (Latinised by Adomnán as filius Cerbulis) the genealogical tradition says that his father's name was Fergus, nicknamed Cerrbél or 'crooked mouth'.  His grandfather Conall son of Niall was nicknamed Cremthainne (possibly denoting fosterage among the Uí Chremthainn of Airgialla), to distinguish him from his brother Conall Gulban, ancestor of the Cenél Conaill.  The habit of giving the same name to different sons remained common among the prolific Irish princes until the sixteenth century.

Reign
The Annals of Tigernach record that Diarmait celebrated the Feast of Tara, his inauguration as King, in 558 or 560. The previous King of Tara, according to the earliest lists, was Óengarb, an epithet meaning "extremely rough", presumed to refer to Diarmait's kinsman Tuathal Maelgarb. What followed the inauguration was "a surprisingly unpropitious reign for so famous a king".

Diarmait was defeated at the battle of Cúl Dreimne (near Ben Bulben in modern County Sligo) in 560 or 561. This was the "Battle of the Books", supposedly the result of Diarmait's judgement in a dispute between Columba and Finnian of Moville. Columba, it is said, had secretly copied a book belonging to Finnian, and the matter of ownership of the copy had come to be settled by Diarmait, who adjudged in Finnian's favour, reportedly saying "[t]o every cow its calf and to every book its copy." Columba sought support from his kinsmen among the Cenél Conaill and the Cenél nEógain of the northern Uí Néill who went to war with Diarmait. This is a late tradition, and annalistic accounts claim that the battle was fought over Diarmait's killing of Diarmait of Curnán, son of Áed mac Echach (d.575), the King of Connacht who was under Columba's protection.

Following this defeat, Diarmait lost the battle of Cúil Uinsen to Áed mac Brénainn, king of Tethbae in Leinster. Diarmait played no part in the great Uí Néill victory over the Cruthin at Móin Daire Lothair in 563. He was killed in 565, probably at Ráith Bec in Mag Line (Moylinny, near Larne) in Ulster by Áed Dub mac Suibni, king of the Cruthin.

According to the later Irish historians, Diarmait was followed as King of Tara by Domnall Ilchegalch and Forguss, sons of Muirchertach mac Ercae, of the Cenél nEógain. More contemporary sources suggest that the Kingship of Tara all but disappeared in the years following Diarmait's death, and that it was not until the time of Domnall mac Áedo, or perhaps of Fiachnae mac Báetáin, that there was a High King of Ireland again.

Saints and Druids
Adomnán of Iona, writing less than 150 years after Diarmait's death, describes him as "ordained by God's will as king of all Ireland." Given that the annals say that Diarmait celebrated the Feast of Tara, the pagan inauguration ceremony, Adomnán's words represent his view of kingship rather than the reality of Diarmait's life. Most traditions portray Diarmait as in conflict with saints and holy men, notably Columba. A later poet has Diarmait say "Woe to him that contends with the clergy of the churches".

A poem, Mairg thochras fri cléirchib cell ("Woe to him who contends with the clergy of the churches") in the Book of Leinster, is ascribed to Diarmait.

Prophetic Death of Diarmait
Supernatural features in Diarmait's reign are not limited to prose and verse works or to lives of saints. Even the Irish annals include a reference to druid fences being created at the battle of Cúl Dreimne. The main subject for later writers and poets however, was not Diarmait's life but his death.

Diarmait is told by Bec mac Dé that Áed Dub, Diarmait's foster-son, will be his killer. Accordingly, Diarmait banishes Áed Dub. Saint Ruadán gives the prophecy that Diarmait will be killed by the roof-beam of his hall at Tara. Diarma