Visited 6th August 2005: This site isn't acknowledged by everyone as prehistoric, but having read about it and spoken to Margaret Ponting about it, I was curious. Cnoc Sgeir na h-Uidhe (pronounced kroc skayth na hoo-ya) is on the way to Na Dromannan:
Starting from the A858, park in the lay-by nearest to Cnoc Fillibhear Bheag. Cross the road and follow it, heading away from Calanais, until you are past the cottages on your left (I think you may cross over a cattle grid, but I'm not 100% sure). Here there's a gate taking you into a small paddock on the other of which is another gate. From this point on you're in open countryside.
At this point work your way onto the rocky outcrop behind the cottages, and this is where you'll find Cnoc Sgeir na h-Uidhe. Don't expect to find it easily!