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In the Ulster Cycle of Irish mythology, Dáire mac Fiachna was an Ulster cattle-lord and owner of Donn Cuailnge, the Brown Bull of Cooley, over which the Táin Bó Cuailnge (Cattle Raid of Cooley) was fought.
Queen Medb of Connacht had discovered that her husband, Ailill, was richer than her to the tune of one extremely fertile stud bull, and resolved to even the account by possessing Dáire's great bull. 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, and Medb did indeed take the bull by force.
Following the medieval Irish genealogies, Dáire appears to have been a paternal relative of Conchobar mac Nessa.
See also 
- Cecile O'Rahilly (ed & trans), Táin Bó Cúalnge from the Book of Leinster, Dublin Institute for Advanced Studies, 1967, pp. 138-141