Diarmuid, now, was son of Bonn, son of Duibhne of the Fianna, and his mother was Crochnuit, that was near in blood to Finn. And at the time he was born, Bonn was banished from the Fianna because of some quarrel they had with him, and Angus Og took the child from him to rear him up at Brugh na Boinne.
And after a while Crochnuit bore another son to Roc Diocain, that was Head Steward to Angus. Roc Diocain went then to Donn, and asked would he rear up his son for him, the way Angus was rearing Donn's son. But Donn said he would not take the son of a common man into his house, and it would be best for Angus to take him. So Angus took the child into Brugh na Boinne, and he and Diarmuid were reared up together.
And one day Finn was on the great Hill at Almhuin of Leinster, and no one with him but Donn and a few of the poets and learned men of the Fianna, and their hounds and dogs, and Bran Beag came in and asked did he remember there were bonds on him, not to stop in Almhuin for ten nights together. Finn asked the people about him then where would he go and be entertained for that night, and Donn said: "I will bring you to the house of Angus, son of the Dagda, where my young son is being reared."
So they went together to the house of Angus at Brugh na Boinne, and the child Diarmuid was there, and it is great love Angus had for him. And the Steward's son was with him that night, and the people of the household made as much of him as Angus made of Diarmuid; and there was great vexation on Donn when he saw that. It chanced after a while a great fight rose between two of Finn's hounds about some broken meat that was thrown to them; and the women and the common people of the place ran from them, and the others rose up to part them from one another. And in running away, the Steward's child ran between the knees of Donn, and Donn gave the child a strong squeeze between his two knees that killed him on the moment, and he threw him under the feet of the hounds. And when the Steward came after that and found his son dead, he gave a long very pitiful cry, and he said to Finn: "There is not a man in the house to-night has suffered more than myself from this uproar, for I had but one son only, and he has been killed; and what satisfaction will I get from you for that, Finn?" he said. "Try can you find the mark of a tooth or of a nail of one of the hounds on him," said Finn, "and if you can, I will give you satisfaction for him."
So they looked at the child, and there was no scratch or mark of a tooth on him at all. Then the Steward put Finn under the destroying bonds of the Druid cave of Cruachan, to give him knowledge of who it was killed his son. And Finn asked for a chess-board, and for water to be brought to him, in a basin of pale gold, and he searched, and it was shown to him truly that it was Donn had killed the Steward's son between his two knees. When Finn knew that, he said he would take the fine on himself; but the Steward would not consent to that, but forced him to tell who was it had done him the wrong. And when he knew it was Donn had killed the child, he said: "There is no man in the house it is easier to get satisfaction from than from him, for his own son is here, and I have but to put him between my two knees, and if I let him go from me safe, I will forgive the death of my son." Angus was vexed at what the Steward said, and as to Donn, he thought to strike his head off till Finn put him back from him. Then the Steward came again, having a Druid rod with him, and he struck his own son with the rod, and he made of him a wild boar, without bristle or ear or tail, and he said: "I put you under bonds to bring Diarmuid, grandson of Duibhne, to his death; and your own life will be no longer than his life," he said. With that the wild boar rose up and ran out of the open door; and he was called afterwards the Boar of Slieve Guillion, and it was by him Diarmuid came to his death at the last.
And when Diarmuid came to his full strength he was given a place among the Fianna of Ireland; and all women loved him, and he did many great deeds, fighting with the enemies of the Fianna and of Ireland; and one time he fought a wild ox through the length of seven days and seven nights on the top of the Mountain of Happiness.