Tuatha De Danann

The truth is not known, beneath the sky of stars,
Whether they were of heaven or earth.”

The Tuatha De Danann or the Tribe of Danu are said to be an ancient race of Gods who once ruled Ireland. They are described as being tall with pale skin and red or blonde hair.
It is said that they arrived in Ireland over 4000 years ago and ruled between 1897BC and 1700BC.  
There are many tales of their battles. No one knows where they came from, but there is speculation amongst historians that they came from Norway or Denmark. 
They are said to have brought with them four treasures - 

• The Sword of Light - it was said to glow and that no one could escape it once it was drawn against them
  
• Lugh's Spear - it was said to heat up and sparks flew from it.
  
• The Dagda's Cauldron - It was said to bring the dead back to life
  
• The Lia Fail - Also known as the Stone of Destiny, it was said to proclaim a new king by letting out a roar.

After the Danann were defeated, the Sidhe led them underground and cast enchantments to hide them. In time they became part of the SIdhe themselves.
So what were the Tuatha De Danann? Were they Gods? Or Aliens? Or humans who had developed futuristic tech?