Visited 15th December 2003: This standing stone (or cairn, or whatever it is) is clearly visible from the road that runs to the west of the lower Cregennen lake. A public footpath runs very close to the stone, but although this is the quickest way to get to it, the route involves climbing a ladder to get over a large dry stone wall (so not great for visitors with limited mobility).
It's a beautiful spot, squeezed into the thin strip of flat ground between the lake and Pared-y-Cefn-Hir. Probably a good place for quiet contemplation, if you don't have a four year old with you.
This stone is listed by the Gwynedd Archaeological Trust as a prehistoric standing stone, but Coflein describes it as the remains of two cairns (this reflects the RCAHMW records). I can also see how it might be interpreted as a natural outcrop.
I've plucked the name of this stone out of the sky because it doesn't appear to have one (Plas Cregennen was a house that stood nearby until 1991 - now marked by a modern carved stone). The eight figure grid reference for Plas Cregennen Stone is SH65941450. This is based on the Gwynedd Archaeological Trust records, but having visited the site I'd say it's much more useful than the four figure reference (at the top of the page), which is a bit misleading.