The Otherworld is a reality that lies parallel to our own. As famously described by James Hutton, the Blessed Isles can be considered an archipelago both on the planet as a string of geographic islands rising from the North Sea and in the realities as a string of human settlements rising through the Otherworld. Unfortunately, modern life has seen the other reality gradually receding due to the effects of technology, often called The Allergy.
The true extent of the Otherworld in its own reality is not known but the extent in the human reality is identical to that of the Blessed Isles. Despite the nearness of the Faireys, Shetlands and Orkneys to Norwegian lands and of the Channel Islands to France, these other States do not claim to have their own boundaries with, or even their own version of, the Otherworld.
Geography in the Otherworld does not match perfectly with the Blessed Isles and there have been expeditions that were able to walk to Dublin from Glasgow, St Kilda from Exeter, Chausey from Cardiff, and Ramsey from Hugh Town. There has also been an expedition that had to row across water for twenty days in order to reach Oxford from Swindon.