The sense I got about the Tuatha de Danaan is that the myth might have a tangential connection to Fourth World history, but is probably distorted and largely just a tool the Danaan families use. (It was recorded in the middle ages not sure how old it even is.)
In real life, Irish mythology displays similarities with other celtic mythology, which would hint to a common origin somewhere in Central Europe before 400 BCE and possibly much older (*). The
Tuatha Dé Danann and their opposition to
Formorians is likely to be related to Ancient greek Olympians/Titans or Scandinavian Aesir/Vanir. The
Tuatha Dé Danann mythos does not appear to include the "new generation of gods" trope that other European pantheons have (again, seemingly from the same Indo-European source). On the other hand, its variation on the "Five Ages of Man" (with six ages, the Tuatha being the fifth) includes many biblical reference that suggest late influences.
It worth noting that early Irish mythos, as far as 2nd or 1st millenia BCE, had to deal with visible neolithic monument such as
Newgrange (built circa 3200-3100 BCE) to which the Tuatha were later associated. So whatever mythos they had associated to those monuments and retained, it was merged to the Tuatha mythos when it formed.
( * Outside of the topic discussed here, that's not completely irrelevant in Shadowrun, as the Knights of Rage criminal organization, under Celedyr tutelage, are uniting people of nubian, welsh, irish, breton and austrian origins on the basis of a common mystical heritage)
Tir Tairngir and Tir Na Nog are both two sides of the same coin as far as descendants from the Blood Wood are concerned. Tir Tairngir focuses on the political, and Tir Na Nog focuses on the spiritual. But the spiritual side of Tir Na Nog is what lead to all the queens up to Alachia, each more prideful and self-centered than the last, until Alachia, in her pride, chose to have her people undergo constant torment than to accept the Rites of Protection and Passage from the Therans. That massive act of blood magic, which corrupted an entire nation (both the land and the people), was born from the same attitudes that are present in Tir Na Nog.
From what I remember of
Earthdawn Bloodwood (not so much), noble families and clans played a major role. Tir Tairngire rite of progression rather seemed designed so as to allow the council of princes to make or break dynasty on a whim. I also always find it telling that they choose to have a High Prince instead of a King or Queen.
On the other hand, Tir Tairngire is also where the (few) named NPC from Earthdawn era reappeared in Shadowrun era, including Alachia (which I believe, was originally intented by
Tir Tairngire authors to be Jenna ni'Faira, before someone dropped the ball, requiring the introduction of Sosan Naerain). To me, Tir na nOg is structurally closer to Earthdawn era Bloodwood, but for some reasons the immortal elves who use to be there went on to create something new in Oregon (or more accurately, to
jumpstart something, with a crowd of twenty-something people, due to elven demographics).
It also worth noting that the way the Seelie Court hide in the metaplans seems like a pretty good solution to the Scourge, which in turn suggest it wasn't available at the time. Which may be the origin of the divide. By establishing in the metaplans, the Seelie Court no longer need Alachia or Theran magic to survive the next Scourge. But meanwhile, immortal elves like Alachia don't need the Seelie Court to hide during the Fifth Age.
I'm considering the possibility that the Tuatha could be "regular" elves that discovered how immortals had manipulated them. If you look at those so-called "great elves" as the bastard offspring of dragons and elves, those elves could pose as "pure", the ancestors of the elven race. Moving into the metaplans would allow them to live as long as their immortal rivals, and pass through the Fifth age (with possibly some of them leaving from time to time, infusing Irish clans, even those of Anglo-Norman origins, with elven genes, explaining that great number of elven birth).