As regards Medb, it is related here: She gathered the men of Erin and led them forth to Cruachan to behold the battle of the bulls. As regards the Brown Bull of Cualnge, it is now recounted in this place: When he saw the beautiful, strange land, he sent forth his three bellowing calls aloud. And Finnbennach Ai (‘the Whitehorned of Ai’) heard him. Now no male beast durst send forth a low that was louder than a moo in compare with him within the four fords of all Ai, Ath Moga and Ath Coltna, Ath Slissen and Ath Bercha. And he lifted his head and he hastened to Cruachan to look for the Brown Bull of Cualnge.
It was then the men of Erin debated who would be fitted to witness the fight of the bulls. They all agreed that it should be Bricriu son of Carbad that were fitted for that office. For, a year before this tale of the Cualnge Cattle-raid, Bricriu had gone from the one province into the other to make a request of Fergus. And Fergus had retained him with him waiting for his treasures and goods. And a quarrel arose between him and Fergus at a game of chess. And he spake evil words to Fergus. Fergus smote him with his fist and with the chessman that was in his hand, so that he crave the chessman into his head and broke a bone in his head. Whilst the men of Erin were on the foray of the Tain, all that time Bricriu was being cured at Cruachan. And the day they returned from the expedition was the day Bricriu rose. And this is why they selected Bricriu, for that Bricriu was no fairer to his friend than to his foe. And he was brought before the men of Erin to a gap whence to view the bulls.
Each of the bulls sighted the other and there was a pawing and digging up of the ground in their frenzy there, and they tossed the earth over them. They threw up the earth over their withers and shoulders, and their eyes blazed red in their heads like firm balls of fire. Their cheeks and their nostrils swelled like smith’s bellows in a forge. And each of them gave a resounding, deadly blow to the other. Each of them began to hole and to gore, to endeavour to slaughter and demolish the other. Then the Whitehorned of Ai visited his wrath upon the Brown Bull of Cualnge for the evil of his ways and his doings, and he crave a horn into his side and visited his angry rage upon him. Then they directed their headlong course to where Bricriu was, so that the hoofs of the bulls drove him a man’s cubit deep into the ground after his destruction. Hence, this is the Tragical Death of Bricriu son of Carbad.
Cormac Conlongas son of Conchobar saw that, and he laid hold of a spearshaft that filled his grasp, and gave three blows to the Brown Bull of Cualnge from ear to tail, so that it broke on his thick hide from ear to rump. ” No wonderful, lasting treasure was this precious prize for us,” said Cormac, “that cannot defend himself against a stirk of his own age!” The Brown Bull of Cualnge heard this– for he had human understanding– and he turned upon the Whitehorned. Thereupon he he rushed at him, so that they continued to strike at each other for a long while and great space of time, till night fell on the men of Erin. And when night had fallen, all that the men of Erin could hear was the bellowing and roaring. That night the bulls coursed over the greater part of all Erin.
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Chapter 28 of Lady Gregory’s Gods and fighting men. From sacred-texts.com