Visited 1st September 2002: I first spotted the Garn Lwyd standing stone on the 1:25000 OS map when I was planning a visit to Hirnant Circle just up the road. The stone is very small, and stands next to a cairn of the same name.
There's no public right of way to the site itself, but the cairn is relatively easy to identify from the road if you have a map (it's almost in line with the telegraph pole that has the number 23 on it). Once you've spotted the cairn you can just about see the stone from the road. It's between the cairn and the perimeter fence, but it's less than a metre high so you'll have to look hard for it in amongst the reeds.
It's worth noting that the OS Landranger map only marks the cairn itself, and although the 1:25,000 OS map has the stone marked on it, it's placed too far north (at least that's the only conclusion I could come to).
Garn Lwyd is the name of the cairn next to the stone, but it's also shared by all the cairns in this group. The word garn means cairn in Welsh and lywd means sacred.