"In that battle, moreover, Nuada's hand was stricken off—it was Sreng son of Sengann that struck it off him— so Dian-cecht the leech put on him a hand of silver with the motion of every hand; and Credne the brazier was helping the leech." - Cath Maige Tuired
"Now Nuada was in his sickness, and Dian-cecht put on him a hand of silver with the motion of every hand therein. That seemed evil to his son Miach. He went to the hand which had been struck off Dian-cecht, and he said ‘joint to joint of it and sinew to sinew,’ and be healed Nuada in thrice three days and nights. The first seventy-two hours he put it over against his side, and it became covered with skin. The second seventy-two hours he put it on his breasts. The third seventy-two hours he would cast white [gap: meaning of text unclear/extent: one word] of black bulrushes when they were blackened in fire. That cure seemed evil to Dian-cecht. He flung a sword on the crown of his son's head and cut the skin down to the flesh. The lad healed the wound by means of his skill. Dian-cecht smote him again and cut the flesh till he reached the bone. The lad healed this by the same means. He struck him the third blow and came to the membrane of his brain. The lad healed this also by the same means. Then he struck the fourth blow and cut out the brain, so that Miach died, and Dian-cecht said that the leech himself could not heal him of that blow." - Cath Maige Tuired http://www.ucc.ie/celt/published/T300011.html
This is the core of the story of Dian Cecht and his son Miach, healers of the Irish Gods. Many people read this story as one of jealousy and petty retaliation, but I tend to see it differently. To me this is a story about the natural order, the right order of society, and the consequences of defying that order.
Dian Cecht is the premier physician of the Tuatha de Danann, called the God of health and Healing Sage of Ireland; he possesses a healing well or cauldron (O hOgain). The first question I asked myself when contemplating this story, is why couldn't Dian Cecht heal Nuada's arm? Of course we could assume that he lacked the skill, but that seems unlikely to me - rather I think it is more likely that the arm was not healed because it was not meant to be. Nuada had been king for 7 years when his arms was lost, meaning he had to forfeit the kingship as only the physically perfect could rule, and then Bres became king. Nuada being restored by Miach allowed the Tuatha de Danann to rebel because Nuada was fit to rule again; however it is worth keeping in mind that Dian Cecht's grandson by his son Cian is Lugh Lamhfada who was also destined to be king, and would indeed take the kingship from Nuada later on. Perhaps - and this is purely my theory - it was not jealousy that motivated Dian Cecht to attack Miach but the knowledge that healing Nuada had changed what would have otherwise happened, which likely would have been Lugh showing up to take the throne from Bres himself. It was Lugh who won the battle for the Tuatha de, and Lugh who killed Balor of the Evil Eye, Balor having - so the story goes - killed Nuada and his wife Macha in the battle. Nuada could not overcome Balor and win the battle but Lugh could and did - a fact Nuada seems to acknowledge to some degree as he allows Lugh to lead during this time. But, Gods never really dying, now the Tuatha de Danann had Nuada as king and Lugh as destined-king. I can see how, if Dian Cecht had any inkling of this, he might see healing the displaced king as a bad idea, something that disrupted the natural order.
Miach looks at his father's replacing of Nuada's arm with one of silver and declares that the cure seems evil to him - perhaps because he knows the arm could be restored - and so he sets out to heal it as he believes it should have been done. Dian Cecht sees the arm restored and declares that that healing seems evil to him - perhaps because he knows it has thrown off the natural order of the kingship - and attacks Miach, wounding him four times with the fourth time being fatal. No worries though as his death is only as permanent as any of the other Gods, and he shows up again later healing the wounded with his sister by his father's side. When I look at the story of Miach and Dian Cecht I see a father who allowed a wound to heal a certain way because he was taking the long view, and a son who stepped in and through arrogance healed the same injury, not because it was right to do so, but to prove that he could do it.
The Lore of Ireland by Dáithí O hOgáin
Cath Maige Tuired http://www.ucc.ie/celt/published/T300011.html