I don't think it is necessary to use extraterrestial theory here, although, it is not altogether out of the question.MEN OF DEA
There are many theory about who were the Tuatha De Danaan an where they came from.
However the second question should not be too mysterious as Lady Gregory write in "Gods and Fighting men" Book one, (4th line):
"It was from the north they came".
Therefore, we have here a people of Nordic origin obviously.
The first few lines say:
"It was in a mist that the Tuatha De Danaan, the people of Dana, or as some called them, the Men of Dea, came through the air, the high of Ireland"*.
"It was on the first day of Beltaine, ..., that the Tuatha De Danaan came, and it was to the North-West of Connaght they landed.
But the Fir-Bolgs, ...saw nothing but a mist... lying on the hills."*
It seems that those people came "through the air". Therefore, unless they were Angels or birds-people, and could fly by themselves, they were using some type of flying machine. I don't think it is necessary to use extraterrestial theory here, although, it is not altogether out of the question.
This picture represent a model or toy aeroplane, which is kept in a museum in Cairo (Egypt).
Other traditions have preserved the memory of flying machine like in South America. 
This represents gold model planes found in Bolivia.
Those objects are thousands of years old and offer no explanation.
I am not saying that there is a connection, but the use of air crafts is not unusual in mythology and that some artifacts seems to confirm this.
___________
*Extract from the book Gods and Fighting Men" by Lady Gregory" Copyrights 1904. Published by Colin & Smithe ISBN 0-901072-37.
____________
