I'm glad y'all liked it. I was worried I was going on a bit! I'm afraid I can talk about Celtic mythology, especially the Irish matter, until just about anybody is ready to cry "enough!"
Solar, one thing about the Celtic Otherworld is that it almost seems to be a different place each time it is visited. It is sometimes called Tir na n'Og (Land of Youth, or Land of Youthful Beauty), and is portrayed as being much like a timeless and supernaturally beautiful Version of our own world.
There are other terms for the Otherworld, such as Tir na mBan (Land of Women), Tir na mBeo (Land of the Living, or the Living Land), Tir Tairnigiri (Land of Promise), and so forth. It varies a LOT from story to story and visitor to visitor.
Really, I think you might want to look more to the Grecian depictions of Hades for an Underworld appropriate to Lunamist. The amazingly beautiful and timeless Tir na n'Og is a great place to visit, but you wouldn't want to adventure there.