Sometimes Monhegan Island seems more like the Celtic mythological island Tír na NÓg but indeed Monhegan Island is real and you can visit it off the coast of Maine.
The village on Monhegan Island, Maine and Manana Island across the harbor.
Note that the remote location and lack of medical facilities can make travel there difficult and sometimes dangerous. Make sure to familiarize yourself with the rules of the island so it can be preserved for generations to come.
For more information visit: http://monheganwelcome.com/