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A ninth century rémscéla or foretale recounts how the tale came to be. In the 6th century, the poet Senchán Torpéist gathered the poets of Ireland together to see if any of them knew the story of the Táin Bó Cúailnge, but they all only knew parts of it. His son Muirgen came to the grave of Fergus mac Róich and spoke a poem, and Fergus' ghost appeared to him and related the events of the Táin as they happened.
He was originally a man named Friuch, a pig-keeper, who worked for Bodb Dearg, king of the Munster sidh. fell out with Rucht was a pig-keeper for Ochall Ochne, king of the Connaught sidh. The two fought, transforming into various animal and human forms, ultimately becoming two worms which were swallowed by two cows and reborn as two bulls, Donn Cuailnge and Finnbhennach ("White-horned"). Donn belonged to Dáire mac Fiachna, a cattle-lord of Ulster; Finnbhennach was born into the herds of queen Medb of Connacht, but considered belonging to a woman beneath him and joined the herds of her husband, Ailill.
The Mórrígan had a heifer which she took to Cooley to be bulled by Donn. The result was a bull-calf which fought Finnbhennach and narrowly lost. After seeing that, Medb was determined to see Finnbhennach fight the bull-calf's sire.
When Medb discovered that owning Finnbhennach made Ailill richer than her, she resolved to even the account by possessing Donn Cuailnge. She sent messengers to Dáire with an extremely generous offer of land and treasure, and if necessary sexual favours, if he would loan the bull to her for a year. Dáire agreed. However the messengers got drunk, and one boasted that if he hadn't agreed Medb would have taken the bull by force. When Dáire heard that he backed out of the deal.
Medb raised an army to steal Donn, and marched on Ulster. As Fergus mac Róich knew the terrain he was chosen to lead. The Mórrígan, in the form of a crow, warned Donn of the coming army, and Donn Cuailnge goes on a rampage.
The men of Ulster were intent on fighting Medb's army but were disabled by the curse of Cruinniuc's wife, Macha. The only person fit to defend Ulster is seventeen-year-old Cúchulainn but he lets the army take Ulster by surprise because he's off on a tryst when he should be watching the border. Medb ultimately tracked Donn Cuailnge down and he gores the first Connaught herdsman that attempts to capture him. He then stampeded with fifty heifers through the camp killing fifty warriors before charging off into the countryside.
Cúchulainn meets Medb's army on the mount of Slieve Foy, who invoked the right of single combat at a ford, defeating a series of champions in a standoff lasting months. While Cúchulainn is thus engaged Buide mac Báin has found Donn Cuailnge and is driving him and twenty-four cows to Connaught. Cúchulainn kills Buide mac Báin and his twenty-four followers, but in the carnage Donn Cuailnge again runs off.
Eventually, after a pitched battle with the Ulster forces, Medb's armies were forced to retreat, but they managed to bring Donn Cuailnge back to Cruachan. He and Finnbhennach fight. After a long and gruelling battle Donn killed his rival. Mortally wounded himself, he wandered around Ireland inspiring placenames before returning to Cooley to die.
The oldest recension of the tale is found in the Lebor na hUidre ("The Book of the Dun Cow"). This was in large part copied in the Lebor Buide Lecáin, ("The Yellow Book of Lecan"), which was completed around 1390.
- Thomas Kinsella, "How the Táin Bó Cúailnge Was Found Again", The Táin, 1969, pp. 1–2
- Murphy, Maureen O'Rourke, and Mackillop, James. "Old Irish Prose", An Irish Literature Reader, Syracuse University Press, 2006, ISBN 9780815630463
- "The Tain Bó Cualigne -The Brown Bull of Cooley", Carlingford and Mourne
- MacKillop, James. "Táin Bó Cúailnge", Myths and Legends of the Celts, Penguin UK, 2006, ISBN 9780141941394
- "Donn Cuailnge", Oxford Index