Tir n'an Og is indeed a Celtic myth reference. It means either "Land of Many Colors" "Land of Eternal Youth" or something similar along those lines. It's one of the "faerielands" or underworld/paradise, along with Annwn (Annwfn), the Welsh underworld, ruled by Prince Arawn. Another is Mag Mell trans "plain of joy", the Irish paradise. The Greeks have Hesperides, the garden at the end of the world, and the Elysian Fields (paradise). The Russians, comparatively, have the island of Buyan, which can mysteriously appear and disappear, and where many strange things occur. Koschei the Deathless--a sort of unkillable troublemaking mischief-god, more in the vein of a dirty old Zeus than a Loki or a Christian Devil--keeps his soul on this island, inside of a needle, which is inside of an egg, inside of a duck, in a hare, in an iron chest, buried under a green oak tree. The Irish and the British also have several "sacred isles", such as Avalon-itself a faerieland, and an island where the sheep are either white or black, changing color as they move from one side of the island to the other.
Pretty much every culture we humans have ever come up with has stories about some form or another of faerie or dwarf, from the house spirit Domovoi of the Russians, to the Norse Duergar, to the Hawaiian Menehune-superb craftsmen of small size who live in the jungle and dine on bananas. Dahomey-once a kingdom in what is now Benin, in Africa-has the Aziza, a beneficient race of faeries who live in the forest and give good magic to hunters. The Aziza also provide practical and spiritual knowledge.
The Fomor (Fomorians) were supposedly the original "race" in Ireland. Misshapen, and often gigantic, they were ruled by the elderly-but nonetheless awesome-Balor "of the evil eye" who could kill by just looking at his foes, but was so enfeebled that he had to be lifted up with ropes and pulleys when it came time to do battle. The Fomorians basically represented chaos and evil, and had been extant on Ireland as a "people" since before the first Partholanian invasion. The Partholanians are devastated by a plague and Ireland is next invaded by the Nemedians. All 8060 Nemedians die mysteriously-possibly as a complication resulting from close interbreeding, as supposedly the Nemedians were descended from one man, Nemed son of Agnoman, four other men and four women. Next to arrive is Semion, son of Stariat, from whom three tribes, the Firboigs, the Fir-Domnan, and the Gailoin--together known as the Firbolgs, are descended. The Firbolgs together seem to represent a less-than-totally-successful race of people, in that they are generally represented as inferior and/or servile. The first real conquerors of Ireland show up next, and they are the Tuatha de Danaan-trans "the folk of the god who's mother is Dana". Now, I've read that they were named that as a corruption of Latin, basically the Romans were saying the Tuatha (people (of Ireland)) were the children of their (the Romans') goddess Diana. We do have a goddess in Ireland though named Brigit (Brigid or Brigido, Christianized into "St. Brigit"), and she may have been one of the principle gods, being the daughter of Daghdu "The Good", another giant who's battle-club was apparently so large that it had wheels on it. Daghdu (Dagda or Dagdu) may have been a holdover god from a very primitive time, as he seems to be the most crudely represented and most basic-but also most primal-of the Irish gods. Now, the Druids and Bards of Ireland tended to represent the Tuatha de Danaan as scholarly warrior-poets who studied the arts as well as science (such as it was in those times) and magic. The "common" people of Ireland, however, revered them as earth-deities over agriculture, fertility, and nature. The Tuatha initially try to make peace and live in harmony with the Firbolg, but the Firbolg refuse and battle the Tuatha on the Plain of Moytura. The Firbolgs lose and are relegated to County Connacht, where people claiming descent from the Firbolg still survive. Nuada of the Silver Hand is king of the Tuatha at this point, but his hand is cut off, and this prevents him from continuing to be king. The Tuatha continue to have problems with the Fomorians, however, who are apparently much more dangerous than the Firbolg, with more numbers, better leadership, and all the advantages of being evil misshapen giants of primordial evil. Lugh of Many Talents then shows up on the scene, ultimately defeating the Fomorians. There's an excellent source of information about this at
http://www.sacred-texts.com/neu/celt/mlcr/mlcr03.htm, from which I borrowed and distilled much of the above information, adding a bit here and there from my own studies. I also did some quick research in the Wikipedia, which is as fine a place to learn something as I've ever come across. Cheers!