The Mushroom Isle is the location of the Land of the Leprechauns and the rat's warren.
Background[]
It is named for the Magic mushrooms which grow there. Although the island may also be known as the Land of the Leprechauns as well (or at least had been part of it).
To the southwest of island, it was bordered by the bend in the Raging River and River Fools known as the Dangerous River. The north side of the island was roughly bordered by the Castle's moat, and the northeast and east side of the island was bordered by a river flowing out of the Beautiful Lake. It's not clear if the 'island' was completely surrounded by water or simply a kind of peninsula of land between various waterways. Due to magical law of "containment" its unclear how big the island or land stretched. The Land of the Leprechauns may have been considered outside of Daventry proper (it at least was considered a separate kingdom at one time).
Following the King's Coronation the river either filled in with sediments or was filled in by the returning people of Daventry. The town of Daventry was built roughly in the spot of the former island and down to the Island of the Gnome. Rumplstiltskin built a new house on the dry river bed of what had been the Dangerous River bend between the former island and south of the castle.
Behind the scenes[]
This is mispelled as Mushroom Ilse on the map. Ironically Ilse is the name of a river in Germany.