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.
Although most of us think of Pluto as the Roman equivalent of the Greek god Hades, it is one of the Greek god’s titles, usually given as Pluton, Wealthy. This referred both to the earth’s fertility and the mineral riches that could be mined from it.
The Moon and Pluto compared to the Earth. This picture was composed from File:Enceladus Earth Moon Comparison.png, File:Full Moon Luc Viatour.jpg, and File:Pluto impression.png, by Tom Reding (Wikimedia)
Julius Caesar wrote that the Gauls believed they were descended from Dis Pater. In his writings, he did not give the local names for his deities, substituting ones his readers would recognize. (This was the interpretatio romana, giving foreign deities Roman names and attributes.)
Dis was originally the Roman god of wealth, fertile soil, and underground riches, who became equated to Pluto, Orcus, and Soranus. The question is, what Gaulish god reminded the Divine Julius of Dis?