There are many different ways to become god of the dead. You can win the job by chance (Hades/ Pluto), you can be cast into the underworld by other gods (Hel), marry into the job (Nergal), or you can be the first person to die.
Donn was one of the invaders known as the Milesians, after their father Mil. He was the warlike one, while his brother Armaigen was the poet/judge. They eventually did take Ireland, but not easily, and Donn never got to enjoy their victory.