Tir Nan Og

Tir Nan Og (according to Tecna) is a small island off the coast of Ireland that isn't marked on any map.

History
Tir NanOg is the home of the Earth Fairies as well as a sovirgen kingdom in ancient times. when the wizards of the black circle started hunting the Earth Fairies they used the island itself to imprison the fairies. When the Winx freed the Earth Fairies from imprisionment it was used as headquarters for the short lived revenge campain against the humans.

Season 4
The island first appeared in a vision in episode 16 but the winx didn't go there untill episode 17 when they went to free the fairies of Earth. The island wasn't seen again untill the winx took the black circle to the island for trial.