The word “Celtic” here is something of a misnomer in that the true Celtic cultures could essentially be described as pre-Roman and continental. However, the word remains in use for those Blessed Isles cultures that recognise a pre-Roman or, more correctly, a pre-Anglo-Saxon origin. While arguably native (Irish tradition recognises the Irish as the last in a long line of settlers and the Brythonic nations have been heavily influenced by their Roman occupation) the surviving nations and groups represent the cultures that have been interacting with the Otherworld reality the longest. This has led to an academic school of thought often referred to as “the Celtic Interpretation”.
Academics and researchers with Unnatural Resources use literature and records to examine the origins of fairy folk, heroic figures and euhumerised deities (see the House of Danu, the House of Llŷr, and the House of Chicol) to track signs of interaction with creatures from the Otherworld as well as trying to tease enough information out of location descriptions to possibly identify an Otherworld equivalent.
However, the typical Otherworld location in Irish and Welsh mythology cannot be placed on a map of our reality. It is often an island or under water, whether at sea or in a lake or a marsh. They can sometimes be reached by holy places in a way that we would identify these places as waypoints. Some of these places have, over the years, become associated with the continental traditions of underworlds or lands of the deads, and mound waypoints generally became places that had to be descended into rather than using the boundary above ground – although this may also have been a reflection of the underwater lands. Although we are at too great a remove to identify any of these individual places with any surety, some of the names used for Celtic Otherworlds have become attached to known places in the Otherworld Reality.