The Underworld was believed to be located at different parts of the sea. The Fomorians dwelled in the House of Tethra; this was a sort of Celtic version of Pandemonium. The fish in the ocean were often regarded as the "cattle from the House of Tethra." Another region in the underworld was the realm of the dead which was called Tech Duinn This realm houses all evil doers. The gods were careful to not let anyone who was "evil" to go unpunished they all went to Tech Duinn The "good" people left to a different place, this place was called Tir na mBeo. In this afterlife they passed onto the realm where they may again meet their loved ones and kinsmen.
These are four regions of the Underworld.
This is a warriors heaven, it is an abode where the casualties of battle are waited apon by beutiful women. The women are manifestations of the Badb Catha spirits, which you could compare with the Norse Valkyries. the slain warriors are allowed to fight every day and rise again the next day, and every night they may enjoy an evening of feasting. Those who are killed during the fight may raise again the next day.
The Celtic Druids thought the world to be disc shaped rather than a sphere the land being surrounded by waters of the oceans which filled the Underworld. Water had magikal powers in the Celtic world view.
Water was used to cleanse away malevevolent spirits among other things. Water was used to encircle and give definate bounderies to the nemeton, or sacred place. this was done via a dug ditch. They made the ditch in a circle shape because they believed the world to be round(but flat), allthough they believed the heavens to be square.
the waters of Andumnos, surround outskirts of the world. These waters were called Morimanrusa which ment "sea of the dead." This could be compared with the river Styx, because at death you are taken across these waters, whitch inturn induce forgetfullness about the life you are leaving to ready you for the Celestial plains.
What we must understand is, there is a certain thing that links us to the Otherworld Realms. That link is the Bile or sacred tree.
It has roots in the Underworlds and has branches reaching high into the heavens. In the teachings of the esoteric Irish Druids, the tree is located over the well of Knowledge. Its hazelnuts dropped into the well where they ere eaten by salmon.This tree links the well and Underword waters to the earth and the heavens.