Yay finally a mushroom island. Quontex, do you remember the coords? Whoever's territroy it's in, may I build a small hut with LOTZ of bowls?
Look at the map. I don't know the coordinates either, but if you go straight west from around the middle of the swamp that is just north of Kaeshar, you'll reach it eventually.
It's a kilometer+ from the nearest claimed land. Besides, it's just been discovered (recently anyway) and nobody has claimed it, so you're free to build what you wish.