Trendhorn was a servant in the court of King Conor MacNessa, who was employed by the king on a spying mission.

Stories of Deirdre:
Conor MacNessa had sent word to the exiles Deirdre and the sons of Ushna that it was now safe for them to return to Ireland. He declared that Fergus MacRoy, his patron and advisor, would be their surety. But on landing in Ireland, the sons of Ushna were separated from Fergus, who had to attend a feast given in his honour. They settled for the night in the house of the Red Brand Knights of Ulster. Conor MacNessa, meanwhile, brooded in his fortress about the young hero, Naoise, who had taken his betrothed wife away. He sent Trendhorn to the house of the Red Branch in order to see what the sons of Ushna were doing. When Trendhorn arrived, the house had been locked up for the night. He therefore climbed a ladder to look in one of the windows. He saw Naoise and Deirdre playing chess together, but as he watched, someone looked up and caught sight of him. Naoise was angered, and seizing a chess piece, he threw it at the spying Trendhorn, putting out his eye. When the servant returned to Conor, the king called his soldiers together and urged them to avenge the wounded Trendhorn, and so the great battle against Naoise and his brothers began, in which they were all tragically killed.

Trendhorn was the catalyst for the long delayed conflict between Conor and the sons of Ushna. He was a loyal and obedient servant, and his wounding was all the excuse Conor MacNessa needed to go to battle.