Another possibility: tur- could be whole, absolute, entire, according to OED2 1915. I can't figure out how a Wikipedia editor got a gloss for "tuar lach" out of the Placename DB, which I too would tend to prefer over OED here, because it's not in their glossary. Unsupported, therefore. Bad Wikipedia.

I wonder what the railroad-builders or their admins found in Turlock, CA, to remind them of County Mayo. And: reanalysis of the "loch" lexeme? Support for it over "lach"? Dunno.