That's what gathering up background information feels like.
At the moment it seems to me that the Sumerians imagined the netherworld to be physically connected to this one. As water drains from a sponge, so the vaporous 'soul' drains from the dead body and seeps into the netherworld. When Gilgamesh loses his ellag and his ekidma the living Enkidu goes down to fetch them, but because he breaks the rules, he is seen by the dead and more importantly by Ereshkigal.
Gilgamesh pleads with the gods, and eventually one opens a hole in the earth and Enkidu comes out of the earth as 'a gust of wind'.
But here in Britain, in what small amounts of mythology I can find, we seem to favour a multiverse rather than a far away; the otherworlds are coexistent, and people cross over into the land of fairy by accident rather than by taking a path.
Not that the land of fairy is necessarily the land of the dead, but both places stand for 'other realms'.
Greek myth (via Homer and Hesiod) contains Mesopotamian themes and elements, likewise Norse mythology borrows heavily from the Greek.
Celtic mythology is another kettle of fish all together. The sources are primarily Roman and Christian: an anonymous scholar in the 11th century wrote down the origins and history of the Irish people from the creation of the world down to the Middle Ages (The book of invasions). The Welsh tales are recorded in the Mabinogion.
In many ways a more authentic work is The Fairy-Faith in Celtic Countries by W. Y. Evans-Wentz (1911).
You can read it here: [LINK]