The Enchanted Isles or Shadowy Isles were an archipelago in Belegaer, east of Tol Eressëa.
In other stories
The Magic Isles are referenced in Tolkien's Roverandom. In the story, written down in 1927, the great whale Uin takes the enchanted dog Roverandom (formerly known as Rover) on adventures through the seas: Uin takes Roverandom through the Shadowy Seas to the Bay of Fairyland beyond the Magic Isles where Rover saw the Mountains of Elvenhome and the light of Faery. Roverandom thought he could see the white glint of a city of Elves on a green hill far away in the distance.
The archipelago was set by the Valar after the Flight of the Noldor to shield Valinor and Tol Eressëa. They had an uncertain number and no charted course could go through them, and as such they ensnared any ships trying to reach Aman without permission. Additionally the seas about them were filled with mists. The isles were enchanted, and anyone who set foot on them fell asleep forever. The creation of the Enchanted Isles and the Shadowy Seas was part of the Nurtalë Valinóreva, the Hiding of Valinor.
However, when Eärendil and Elwing came upon the Enchanted Isles in Vingilótë they passed through successfully. This may be attributed to the Silmaril that Eärendil wore upon his brow as they sailed into the West.
The isles were apparently either removed from Arda or at least made less sinister after the War of Wrath, as Tol Eressëa could be seen from Meneltarma on Númenor without problems and no obstacles are mentioned when the Great Armament reached Eldamar. Certainly after the Downfall of Númenor they were gone.