I think this was a clever observation regarding the use of "i t˙aim" in the description of the Coire Brecain and the three streets in T˙aim Drecan. Still, I don't think that this
has to do anything with a possible meaning "vortex" of "t˙aim", but that it is rather due to chance.
DIL says that "t˙aim" is attested in a range of contexts which suggest anything from "mound, hillock, top, bank of a river, enclosure (=rampart?), enclosed area". All of
these meanings can be derived from a basic meaning like "protuberance, swelling", which is also suggested by the most probable etymology (PIE *tewH- "to swell"), but
the can't be derived from "vortex, bending".
As to "i t˙aim araile" in the description of the Coire Brecain, this may admittedly have "whorlly" semantics, but I think we can just as well find a satisfying solution with the
semantic range mentioned in the previous paragraph. "T˙aim" can evidently mean "top, highest point", e.g. "cethair traigid fichet fott in damasnai, ocht traigid eter a thuaim
7 talmain" "the ox-rib was 24 feet long, 8 feet between its peak (t˙aim) annd the ground" (DIL T 335.78). Starting from this meaning, "i t˙aim araile" can easily be
understood as "on top of the other", something which describes well the movement of the waters in the whirlpool. Note that the movement is immediately afterwards
compared to "oirceil tairechtae" "paddles of mill-wheels".
Karin StŘber, The Historical Moprhology of n-Stems in Celtic, p. 68, is cautious about the etymology of the word, but she favours PIE *tewH- "to swell".