Tir Nan Og

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

History
Tir Nan Og is the capital of the Earth Fairies and was as well a sovereign kingdom in ancient times. All the Major Fairies and other Fairy Queens, like Diana - Queen of the Amazon Fairies - and Aurora - Queen of the Arctic Fairies, of Earth take orders from the Fairy High Queen of Tir Nan Og, except, till some extent, Sibylla who, as Fairy of Justice, has to remain neutral and treat friends and enemies, and Good and Evil people all as being equal and cannot take sides. The fact that even Fairies of Tir Nan Og, under Morgana's direct orders, are not allowed to enter her cave when she has provided shelter to the Wizards of the Black Circle. It confirms that Tir Nan Og's power has limitations there. According to Morgana, Tir Nan Og was, before the arrival of Ogron and his Wizards, a wonderful realm located between the Magic Dimension and the human world (now the non-magical Earth), with magnificent buildings and sceneries and the Earth Fairies had contacts with all the other magical realms at this time. Later 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 imprisonment, it was used as headquarters for the short lived revenge campaign against the humans.

Season 4
The island first appeared in a vision when the Winx used their Tracix wings in episode 16, but the Winx didn't go there until episode 17, when they went to free the fairies of Earth. The island wasn't seen again until episode 24, when the Winx brought the Wizards of the Black Circle there for the trial. When Morgana put the Black Circle on her finger during the ceremony, as a sign of the capitulation and the submission of the Wizards to the justice of the Fairies, the Black Circle released dark energy which created a vortex in a large crack in the ground, which led to the Dark Abyss and would suck up all of the Earth fairies, sealing them away for good. Luckily, the Earth fairies were all saved, and Nabu was able to close the chasm. Sadly, the dark magic had drained Nabu of all his energy, possibly resulting in him ending into a coma-like state (Morgana said that she would take care of him till he wakes up again).

Trivia

 * The original island of Tír na nÓg, meaning the Land of Youth", features in Irish mythology as the place where the Tuatha Dé Danann, the previous inhabitants of Ireland (originally Divine and considered to be Gods, but later came to be regarded as Fairies), settled after the humans (the Gaels - the ancestors of modern Irish people) arrived in Ireland. It was said to be located in the Ocean far in the West, beyond the edges of the World and could be reached only after a difficult voyage or by being invited by a messenger of the Gods, that is of the Island's inhabitants, a Banshee (Banshees are fairies regarded in Irish mythology as messengers of the Otherworld and omens of Death, and act as messengers of the Gods to the humans).
 * The Island of Tir Nan Og in Winx Club may also be having references from the mythical Island of Avalon (the Island of Apples) of the Arthurian Legend, which was usually associated with Morgan le Fay (who possibly inspired the character of Queen Morgana), who was supposed to live there with her eight sisters, as references to the Arthurian Legend have already been made when Faragonda was first telling the Winx Club the story of the Wizards of the Black Circle. According to legend, it was an enchanted island which would produce good things by itself such that it was not necessary to cultivate the soil, and fruits, cereals and other good things would grow by themselves. There, people would live a hundred years (which was unusually long in medieval times). It was supposed to be the place where King Arthur's sword Excalibur was forged, and where he was brought to rest following his final battle with his evil son Mordred and his apparent death, since it was said that he will raise again from the dead to lead his people against their enemies.
 * The original Irish name is written Tír na nÓg and it is spelled "Teer na n-oh-g", unlike in the series where is is spelled "Tur nan Og".