The god of healing and head doctor to the Tuatha Dé Danann. Sometimes he is also portrayed as a god of craftsmanship. He is the son of the Dagda, and father of Airmed, Octruil, and Miach.
Dian-cécht and his children created the well of Slaine (sometimes spelled the well of Slane), which would resurrect dead warriors and heal the wounded.
When the king of the Tuatha Dé Danann, Nuada, lost his arm in battle, Dian-cécht attempted to heal it by making a silver prosthesis. Since the king of the Tuatha Dé Danann was required to be 'whole', Nuada could not rule with his silver arm*, and was replaced by Bres. Bres turned out to be a bad leader, and was overthrown. Luckily, while Bres was making a mess of things, Airmed and Miach (also healers), healed Nuada's arm completely. Dian-cécht killed Miach in a jealous rage.**
* In some versions, the silver arm was sufficient, and Nuada took the throne back from Bres as soon as it was in place. But even in this version Dian-cécht kills Miach after he and Airmed heal the arm.
** It is sometimes said that Dian-cécht didn't mind them healing the arm, but the method they used was unacceptable. I don't know what this method was.