[snip everything]
Ok, how's this for a more plausible theory?
First, lets start with known facts:
1) Dwarven legends tells that spiders lived deep in the caves around their mountain homelands.
2) The Dwarves, for whatever reasons, are brilliant engineers.
3)The Smiths of Muirthemne built the Tain for Connacht as a weapon against the Myrkridia.
4) The Smiths of Muirthemne worshipped spiders, and theirs shrines were eventually assailed by Connacht, though the Smiths and their followers were nowhere to be found.
5) There are ways to exit the Tain, but they are protected by traps and spiders.
6) Only Dwarves can pick up the magic crystals inside the Tain to pass through magic gates and the exit.
7) Something strange happened inside the Tain, with warriors souls being taken to some other realm, and spiders crawling out of the dead warriors' skulls.
So, here's my theory:
There are gods who rule the spiders, or who are spiders themselves. These reside in another realm, or perhaps somewhere deep beneath the earth. Perhaps it was with their guidance that the Dwarves became so technologically marvelous. Either way, the Dwarves came to worship the spiders or spider-gods. They sacraficed souls to these spider-gods, which perhaps was required for the spiders to breed new queens or some such.
The Dwarves were a key aspect of the Cath Buig Empire. They continued their worship of spider-gods as they were assimilated into the creater Bruig culture. With their technical genious, they became the official Smiths of Muirthemnem. Then came the Myrkridia, and later Connacht.
Connacht commanded the Smiths to create a self-contained pocket-universe, compressed along all dimensions so that the huge caverns within would fit in the palm of the hand, and that aeons within were hours without. Incase it were every used against them, they built an exit that could only be opened by a Dwarf with three magic crystals, each of which was guarded behind a lightning tower and hidden in the nest of a spider queen - and the whole exit area required the sacrafice of four men to get to (ignoring the bug in "Heart of the Stone"). To keep the spiders inside alive and well, they also built temples where they could sacrafice people to the spider-gods, allowing them to breed their queens or whatever.
So the Tain was used against the Myrkridia, who weren't intelligent enough to escape, and the lightning towers protected the spiders from them. The Myrkridia cannibalized eachother and eventually died out. In the outside world, meanwhile, Connacht could no longer ignore the atrocious sacrafices commited by the Smiths, so he assailed their shrines - but they were long gone, having taken themselves and their followers into the Tain. What they did from there, we may never know. Perhaps, as the Deceiver was able to do, they went inside at one point and then exited at another, going back to Myrgard or Stoneheim. Perhaps they died inside, prematurely or of old age. Perhaps the barbarians who stole the Tain were instructed to do so, so that the Smiths could exit later in the land the barbarians took the Tain to.
Whatever happened to them, a thousand years later Soulblighter found the Tain. He trapped the Legion inside, but they were able to sacrafice four men to get to the exit area, outsmart the traps and kill all the spiders, use their Dwarves to escape, and even to shatter the Tain from within. While inside, however, a number of men were lost to the sacraficial temples.
Sixty years after that, Soulblighter used a shard of the broken Tain to let the Summoner ressurect the Myrkridia; the Deceiver and the Legion (again) went inside and killed the Summoner, using Dwarves again to get past magic gates (how did the Summoner get past them?). Then they teleported out and wound up a half a continent away (how?). Soulblighter and the Deceiver both died. What happened to the Tain shards? Where are the rest of them? We may never know
