Tir Nan Og
There are no maps depicting this mythical island. It comes from the Irish oral tradition. The following is an excerpt describing it from Fairy and Folk of the Irish Peasantry, edited and selected by W.B. Yeats, 1888:
There is a country called Tír-na-n-Og, which means the Country of the Young, for age and death have not found it; neither tears nor loud laughter have gone near it. The shadiest boskage covers it perpetually. One man has gone there and returned. The bard, Oisin, who wandered away on a white horse, moving on the surface of the foam with his fairy Niamh, lived there three hundred years, and then returned looking for his comrades. The moment his foot touched the earth his three hundred years fell on him, and he was bowed double, and his beard swept the ground. He described his sojourn in the Land of Youth to Patrick before he died. Since then many have seen it in many places; some in the depths of lakes, and have heard rising therefrom a vague sound of bells; more have seen it far off on the horizon, as they peered out from the western cliffs. Not three years ago a fisherman imagined that he saw it. It never appears unless to announce some national trouble.
There are many kindred beliefs. A Dutch pilot, settled in Dublin, told M. De La Boullage Le Cong, who travelled in Ireland in 1614, that round the poles were many islands; some hard to be approached because of the witches who inhabit them and destroy by storms those who seek to land. He had once, off the coast of Greenland, in sixty-one degrees of latitude, seen and approached such an island only to see it vanish. Sailing in an opposite direction, they met with the same island, and sailing near, were almost destroyed by a furious tempest.
According to many stories, Tír-na-n-Og: is the favourite dwelling of the fairies. Some say it is triple-the island of the living, the island of victories, and an underwater land.