"cnoc" is usually pronounced "knock" - in some cases the name has even changed to the spelling "knock" (as in the Knock of Crieff http://www.streetmap.co.uk/streetmap.dll?G2M?X=286445&Y=722790&A=Y&Z=3)

but it can also be pronounced "crock" in some cases. I can't think of any other "random" Gaelic pronounciations for megalithic sites off the top of my head, but I can think of plenty of placenames that sound very different to their spelling, if anyone's interested?

Cheers
Andy