I don't think she's ever been confirmed as a 4th worlder, and as i said the way the she acts in "Worlds without end", she seems to have most of her information about anything really from the likes of Alachia. Thus i've always seen her as a 6th worlder IE, WHO has been tutored in the old ways, or at least in the old ways that people like Alachia and the other old IE's in Tir na Nog felt that she needed. It is very clear though that there are huge holes in her knowledge.
In my mind i've always thought that she was as i said a compromise because she isn't dangerous to any of the really old ones, so essentially they reestablish the old ways ( Elven Court, path of the Wheel..etc. ) but without loosing much of their own power because of a strong queen like back when Alachia held the Throne before the scourge. Alachia's history though also explains why there would be strong forces in the Tir, WHO would rather see anybody else on the throne than her. However it is clear that she has wormed her way into the circle of power by acting as Lady Brane Deighs personal tutor and advisor, which Again shows Brane Deighs inexperience i think, as anyone with direct 4th World knowledge would be extremely wary of Alachia ( Blood Queen and all that ).
But Brane Deigh as a considerably younger 4th worlder than the others could be a possibility as well, i am still leaning towards the version of her a 6th worlder personally though.
The problem is of course, that while we know quite a bit about the IE's behind Tir Tairngire ( some of them at least ), we don't know all that much about the ones behind Tir na Nog. Though i seem to recall from the old Tir na Nog sourcebook, that even amongst the Danaan families, there are some families, that are considered more important, the Danaan Mor if i recall correctly. My bet is that at least the Danaan Mor families are founded by an IE. Which would mean that there's maybe a bit more but at least the same number of IE's in Tir na Nog as in Tir Tairngire