Some of the Tuatha de Danann were found in stories to inhabit mysterious islands, like the isle of Emain Ablach (Avalon) . . .

There, it is said -

Pain and treachery are unknown, so, too, are grief, mourning and death, disease and infirmity, that is the song of Emain . . .

What a marvellous land is this! The young do not grow old at all . . . Here is a bough from the apple tree of Emain that I bring you like the others; it has twigs of white silver and brows of crystal with flowers.

The Tuatha de Danann inhabited the "islands at the north of the world" and it's likely the same land where we find the Hyperboreans - the ancient inhabitants of Ireland. The Tuatha de DAnann were conquered by the Gaels and made into slaves. Some fled to the barren lands where other peoples minds turned them into the sidh. These became the fairy mounds that were really dolmens and tumuli. As time went by, they became fairies and/or people of the dark world. Even later, they became associated with the islands and were reinvented in the tales of Avalon/Arthur.

P.S. I had pictures to go with this, but I can't port them over for some reason. I can't seem to link to Photobucket either. Sorry, it's much better with the pictures. Love and Light Brigit