can't remember how I left the island or how to return to it now that the quest is over and mira is in the castle talking about what to do with the crystal...
You need to go to Havaria Port, north west in Central, next to the academy where you start the game. In the top left of the port was Maria to bring you to the island initially. Now, after finishing the events, one of her angel bodyguards should stay at that area and allow you to get back to the island.