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http://www.gods-heros-myth.com/tuatha.html

Very interesting reading.

The Tuatha D? Danann ("People of the Goddess Danu") were one of the
mythical races who settled in Ireland before the arrival of the
Milesians, the ancestors of modern Gaels. The Dananns were descendants
of the goddess Danu. Her son Dagda was their most powerful leader of the Dananns.
Now the Sidhe or People of the Mounds are said to be or be descended of the Tuatha De Danann. Also know as the "Otherworld" was a domain of Celtic deities or supernatural beings such as the "Fairy People". They were hidden from mortal eyes by strong Otherworld magic. They were
situated in all sort of places. Some of these Otherworlds were located
on the islands, the dunes, dun-hills, forests, rivers, and lakes. A
grand castle or even humble cottage could be the Otherworld, which
would, appeared at night for mortals, but would probably vanish in the
morning. The Otherworld also seemed to be able to move from one location to
another. Or there may be only one Otherworld but it exist everywhere.
In another word, the Otherworld is a paradox. Entering the enchanted
place, may be close by or it could be a place far away.

Originally, the Otherworld, particularly the Irish myths, was sometimes
situated on some remote islands in the west. Later the Otherworld was
located on Ireland itself.