It can be difficult to interpret ancient mythology, legends and manuscripts into modern thinking but the two Battles of Magh Tuireadh of Celtic Mythology seem to indicate that the Fomorians and especially the Tuatha Dé Danann were in actuality Hollow Earth inhabitants.

Of all the people I have met and talked to that do indeed subscribe to the Hollow Earth theory, many are divided as to the altruism of its inhabitants and some believe that those that live inside our planet are demons with nefarious intent and because of this none should be trusted. However I submit to my reader that, just as there are evil men with corrupt intentions on the surface, so too are there different factions within our planet that may oppose each other's philosophical beliefs.
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I believe the ancient mythology of the Celts shows that this is exactly what happened however what they mistook as Gods were actually inner Earth inhabitants that had moved to the surface for their war-like nature. Having left through the Northern polar entrance to Hollow Earth in ships (what we would call UFOs) they decided to follow their ancestors the Nemed to Ireland.
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What is interesting to note is that the first stories of the Tuatha Dé Danann has them landing on the mountain tops of Conmaicne Rein (Connachta) and with them they brought three days of darkness. The significance of this is that ancient 'Great Flood' (Polar Shift) mythology the world over speaks of three days of darkness. There is sufficient evidence to show that they arrived in Ireland soon after a major catastrophic flood of global proportions. This would coincide with the passing of Nibiru Planet X 3,600 years ago.
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They were said to have come from four ancient cities far to the North Falias, Gorias, Murias and Finias where they learned their magical skills and knowledge. However since these cities have never been found, then they could very well exist in a land not recognized by our own historians, such as Hollow Earth.