A king of Ulster dies of hearing about the Crucifixion, seven years after being shot in the head with a preserved brain.
That sentence is accurate, and it takes some unpacking. It is also, for a modern reader, the single best demonstration of what medieval Irish scribes were doing with the pagan material they had inherited.
Getting the throne by contract
His mother is Ness, and the standard account of his accession has nothing supernatural in it at all. Fergus mac Róich, then king of Ulster, wants to marry her. She agrees on one condition: that her son hold the kingship for a year, so that his descendants can be called the sons of a king.
The Ulstermen consent, thinking it a formality. Ness then spends the year advising her son and distributing his wealth among the nobility so effectively that when the term expires the province refuses to hand the kingship back. Fergus, who traded a throne for a marriage, has to live with it — and his resentment is one of the standing tensions of the whole cycle.
Some versions add a supernatural conception involving Cathbad the druid and a drink from a river containing two worms; the political version above is the more interesting one and is well attested. The two coexist in the manuscripts without embarrassment.
Emain Macha and the Red Branch
His court is Emain Macha, and the texts describe three houses within it, one of which — the Cráebruad, the red branch — gave English writers the phrase "Knights of the Red Branch". That phrase is a modern coinage layered over an Irish building name; the Ulster warriors are not an order of knighthood and nothing about them is chivalric.
Conchobar's own portrait is mixed in a way that later retellings rarely preserve. He is generous, presides over a court of formidable men, and is treated by the Táin as a legitimate ruler whose province is genuinely wronged. He also claims the right to sleep with every marriageable woman in Ulster before her husband does, which the texts state flatly as a royal prerogative, and his handling of Deirdre is the cycle's clearest case of a king using the law as an instrument.
That episode costs him Fergus and the sons of Uisliu, and sends his best warriors into exile with Medb. The Cattle Raid of Cooley is fought against an army stocked with his own former household, which is a consequence he arranged himself.
Lying incapacitated while a boy holds the border
During the raid he is out of action along with almost every other Ulsterman, laid low by the pangs that fall on the province in its hour of need, so the defence falls to Cú Chulainn alone. When Conchobar finally rises, it is to lead the muster at the end and to take part in the final battle — the great set-piece description of the Ulster host assembling is one of the Táin's most celebrated passages.
His position in the cycle is therefore odd: he is the king everything happens around, who is absent from the action for most of its length, and whose decisions cause much of it.
The brain-ball
Irish warriors, according to the Ulster Cycle, took the brains of defeated champions, mixed them with lime and rolled them into hard balls to be kept as trophies. That is the tale's premise, and whether it reflects a real practice or a literary exaggeration of head-taking is not resolvable — head-hunting among Celtic-speaking peoples is genuinely attested by classical writers and by archaeology at sites in southern Gaul.
Cet mac Mágach, a Connacht warrior, steals the brain-ball of Mes Gegra, king of Leinster, and slings it into Conchobar's forehead. The physician Fingen judges that removing it would kill him, so it is sewn in with gold thread and the king is put under permanent restrictions: he must not ride a horse, must not exert himself, must not have intercourse, and above all must not lose his temper.
He lives another seven years under those terms. It is one of the earliest sustained accounts of a head injury and its management in European literature, and the clinical framing — a foreign body left in place, a stitched wound, a regime of avoidance — is unusually concrete.
The most transparent Christian graft in Irish literature
Then the sun darkens in the middle of the day. Conchobar asks his druid Bacrach what is happening, and is told that at this moment, in the east, Jesus Christ is being crucified.
The king asks what the man had done, is told he was innocent, and flies into a rage on his behalf — drawing his sword and hacking at a wood, saying that this is what he would do to the men who killed him. The exertion bursts the ball out of his head, and he dies. And because he died in defence of Christ, he is one of the two men in Ireland who believed before the faith arrived, and he is saved.
Nobody should read that as ancient tradition. It is a medieval solution to a real problem: how does a Christian scholar preserve, copy and enjoy the deeds of an unbaptised king without consigning him to hell? Ireland produced several such devices — the survivors of the Fenian band living long enough to meet Saint Patrick is another — and this is the boldest of them, arranging a synchronism between the Ulster Cycle and the Passion so that the pagan past can be redeemed at the last possible moment.
It also fixes the chronology. Placing Conchobar's death at the Crucifixion is what puts the whole Ulster Cycle in the first century, a date that has been repeated ever since as if it were evidence.
Was he a real king?
There is no independent evidence for him and the framework that dates him is theological rather than historical. What is real is Navan Fort, the site the texts call his capital, which was a major ceremonial centre; but its most spectacular phase is late Iron Age, and nothing found there names anybody. The Ulster Cycle preserves a memory of a political order in which Ulster was much larger than the later kingdom of that name, which may be genuine, and gives it a cast list that probably is not.
Why is he so unsympathetic in modern retellings?
Because the modern audience arrives through Deirdre. The Revival playwrights made her the protagonist, which makes Conchobar the antagonist, and twentieth-century adaptations have followed. The medieval position is more balanced: the same king is the wronged party in the Táin, the centre of a court the literature admires, and the recipient of a salvation the scribes clearly wanted him to have. A tradition that goes to the trouble of getting a man into heaven is not treating him as a villain.
What happened to Cet mac Mágach?
He is one of the finest secondary characters in the cycle, a Connacht champion who appears at the feast in Scéla Mucce Meic Da Thó and demolishes the claim of every Ulster warrior in turn by naming an injury he has done each of them. He is eventually killed by Conall Cernach. The brain-ball is his most consequential act, and it is delivered years before it takes effect — a weapon that works on a seven-year delay is a very good image for how vengeance operates in this literature.