Donn Cúailnge (‘the Brown One of Cooley’) was no ordinary bull but a supernatural creature with a complex mythological origin. According to the earliest tellings in the Ulster Cycle, the brown bull and its Connacht counterpart Finnbhennach (‘White-Horned’) began their existence as two divine swineherds in the service of the gods who fell into bitter quarrel. The two enemies passed through a chain of animal and supernatural reincarnations — ravens, stags, warriors, phantoms, maggots swallowed by cows — and were finally born as bulls. As Donn Cúailnge, the Brown Bull belonged to Dáire mac Fiachna on the Cooley Peninsula in Ulster (modern County Louth); Finnbhennach had migrated west and joined the herds of Ailill, king of Connacht.
The story
The bull’s powers were extraordinary: he could serve fifty heifers a day, his bellowing filled the valley with sound, a hundred warriors could shelter in his shadow, and his mere presence repelled spectral beings. It was Queen Medb’s desire to match Donn Cúailnge against Finnbhennach — whose matching in size to Ailill’s bull had made her jealous — that provoked the great invasion of Ulster described in the Táin Bó Cúailnge, the central epic of the Ulster Cycle.
After the war the bull was finally brought to Connacht, but Finnbhennach challenged him immediately. The two fought with apocalyptic ferocity across the landscape of Ireland — the noise of their hooves and horns audible across all provinces — until Donn Cúailnge killed his rival and dragged pieces of Finnbhennach’s body across the country in his horns, with place-names formed at the spots where each part fell. Exhausted and mortally wounded, the Brown Bull returned alone to the Cooley Peninsula, bellowed three times, and fell dead. He is documented in the Wikipedia article on Donn Cúailnge, the Irish Gods and Goddesses mythology site, and the Eel & Otter Celtic studies blog.