Conaire Mór ("the great"), son of Eterscél, was, according to medieval Irish legend and historical tradition, a prehistoric High King of Ireland. The early saga Togail Bruidne Da Derga ("the Destruction of Da Derga's Hostel") tells the story of his life from conception to death. Other sources tell very different stories.
According to Togail Bruidne Da Derga, Conaire's mother, Mess Buachalla, was the daughter of the former High King Eochu Feidlech and the fairy woman Étaín. After Eochu died Cormac, king of the Ulaid, married Étaín, but ordered her daughter to be thrown into a pit. His servants instead took the girl to a herdsmen of Eterscél, king of Tara, who brought her up in a house with no door, only a window and a skylight. She grew up to be very beautiful, and when Eterscél's servants became aware of her they brought news of her to the king, who ordered them to seize her so that he could marry her. Before that could happen a bird flew in through the skylight, took human form, and warned her that Eterscél's people were coming for her. He conceived a child with her, and told her to name him Conaire. Eterscél and Mess Buachalla were married, and Conaire was brought up as Eterscél's son. He was brought up alongside three foster-brothers, Fer Lé, Fer Gar and Fer Rogain, sons of Dond Désa, who was a chief of fianna, landless warriors, hunters and bandits.
According to the Lebor Gabála Érenn He took power after killing his predecessor, and his father's killer, Nuadu Necht. In Togail Bruidne Dá Derga he succeeded Eterscél directly. When Eterscél died, a bull-feast was held. A bull was killed, a man ate his fill of its meat and drank its broth, and slept as incantations were chanted over him. Whoever this man saw in his sleep would be the new king. He saw a naked man coming along the road to Tara with a stone in his sling. The young Conaire, meanwhile, was hunting birds in his chariot. He chased them into the sea, where they became armed men, announced themselves as Conaire's father's bird troop, and informed him that it was forbidden for him to hunt birds. The leader of the bird troop told Conaire to go naked to Tara, where he would be made king, and placed several geasa (taboos) on his reign. Among other things, he may not be preceded by three red men into the house of a red man. As he approached Tara he was met by three kings carrying clothes for him, and when he arrived he was made king.
Another saga, De Shíl Chonaire Móir ("Of the Seed of Conaire Mór"), contains a different version of Conaire's accession. After Eterscél's death, the Laigin and the descendants of Conn Cétchathach gather at Tara to choose a new king. A chariot was harnessed to two unbroken horses, which it was said could only be controlled by the rightful king; in the chariot was a cloak which would only fit the rightful king; two stones, Blocc and Bluigne, would part wide enough to allow a chariot through when it was driven by the rightful king; and finally the Lia Fáil or Stone of Destiny would screech against the chariot's axle when it was driven against it by the rightful king. The first candidate to succeed Eterscél, Lugaid Riab nDerg, had failed these tests.
Conaire, advised by Mess Buachalla of what was happening, arrived at Tara at the head of a fairy army provided for him by his mother. He was able to control the chariot, the cloak fit him, the stones admitted him, and the Lia Fáil cried out against his axle, and he was acknowledged as Eterscél's son and made king. The fairy army left him, placing on him the injunction that the sun should neither rise nor set on him in Tara.
The events leading to Conaire's death are told in Togail Bruidne Da Derga. His reign was long (thirty or seventy years, depending on the source consulted), prosperous and lawful, but his former foster-brothers turned to their father's trade of banditry. Every year, they stole a pig, a calf and a cow from the same farmer, to see if Conaire would punish them. When the famer complained to the king, Conaire told him to take it up with the sons of Dond Désa himself. Encouraged, the brothers' campaign of plunder extended throughout Ireland. Eventually Conaire exiled them to Scotland, but there they made an alliance with Incgél Cáech, king of Britain, and returned to Ireland to plunder it with a larger force. As Conaire pursued them, circumstances contrived to have him break his geasa one by one.
Travelling along the south coast of Ireland, having already broken several of his geasa, he was advised to stay at Da Derga's hostel, but as he approached it, he saw three men dressed in red riding red horses arriving before him. "Da Derga" means "red god". He realised that three red men had preceded him into the house of a red man, and another of his geasa had been broken. The sons of Dond Désa then attacked the hostel. Three times they attempted to burn it down, and three times the fire was put out. Conaire, protected by his champion Mac Cécht and the Ulaid hero Conall Cernach, killed six hundred before he reached his weapons, a further six hundred with his weapons. He asked for a drink, but all the water had been used to put out the fires, so Mac Cécht travelled across Ireland with Conaire's cup, but none of the rivers would give him water. He returned with a cup of water just in time to see two men cutting Conaire's head off. He killed both of them. Conaire's severed head drank the water and recited a poem praising Mac Cécht. The battle raged for three more days. Mac Cécht was killed, but Conall Cernach escaped.
The next High King in the traditional list is Lugaid Riab nDerg. Geoffrey Keating's Foras Feasa ar Éireann has him succeed Conaire directly, but the Lebor Gabála Érenn and the Annals of the Four Masters have him succeed after an interregnum of five or seven years respectively.
The Lebor Gabála Érenn synchonises Conaire's reign with that of the Roman emperor Augustus (27 BC - AD 14), and makes him contemporary with legendary provincial kings Conchobar mac Nessa, Cairpre Nia Fer and Ailill mac Máta and hence the events of the Ulster Cycle. Togail Bruidne Da Derga, by including Conall Cernach and Maine Mílscothach, likewise makes Conaire's reign contemporary with the events of the Ulster Cycle, which in other stories are synchronised with the life of Christ. The chronology of Geoffrey Keating's Foras Feasa ar Éireann dates his reign to 63-33 BC, that of the Annals of the Four Masters to 110-40 BC.
The traditional list of High Kings places Conaire more than a century before Conn Cétchathach, but as we have seen, De Shíl Chonaire Móir places him later than Conn. In the traditional list, Conn is succeeded by his son-in-law Conaire Cóem ("the beautiful"), son of Mog Láma, whose sons by Conn's daughter Saruit are Caipre Musc, Cairpre Baschain and Cairpre Rigfhota, ancestors of several Munster dynasties. But in the sagas De Maccaib Conaire ("Of the Sons of Conaire") and Tucait Innarba na nDéssi ("The Reason for the Expulsion of the Déssi"), these three are the sons of Conaire Mór and another Saruit, and Conn's Munster rival Mug Nuadat is a son of Eterscél. In several texts Conaire Mór is also known by the epithet Cóem. It seems certain that the two Conaires were once the same figure, split in two to accommodate two incompatible chronological traditions.