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").

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.

[4] Cúchulainn met Medb's army on the mount of Slieve Foy and invoked the right of single combat at a ford, defeating a series of champions in a standoff lasting months.

[3] While Cúchulainn was thus engaged, Buide mac Báin found Donn Cuailnge and drove him and twenty-four cows to Connaught.

[4] Julius Caesar, in his Gallic Wars, refers to a Gaulish ally called Valerius Donnotaurus, an interesting Celtic parallel to the Donn Tarbh of Cooley.