Tír na n-Óg

Tir na n-Og was a world at the end of the universe.

Tír na n-Óg was created by Goibhnie, who wanted to see the sociological effects of making a world. On the world there were creatures like those of the myths of Earth, such as the Unicorns, there known as Firbolg. There was a link between Tír na n-Óg and Earth due to a Stone Circle on Earth being a gateway. This allowed many humans to go to Tír na n-Óg by accident. At one point the Doctor and Ace went there by accident and restored the sun, there known to locals as Arawn’s Wheel. Once the Doctor had saved it, he closed of the portal and gave the Arawn's Wheel 2,000 more years. (NA: Cat's Cradle: Witch Mark)

Behind the scenes
According to Irish mythology, Tír na n-Óg is the Land of Eternal Youth where no man ages.