Visited three times in the last week and the cotton grass is looking particularly nice up there at the moment. I've posted a few images including this which were captured as part of my ongoing visits to the stone (https://twitter.com/search?q=%23maenllia2015&src=typd&vertical=default&f=tweets)
This may be much more accessible in the future as they are currently starting work clearing the forest here. There were working diggers within 100m of this site creating a new forest clearing and if you use this new track you can get to the cairn without any risk of wet feet.
I have visited this site several times in the last year and is relatively easy to find from the closest road if you have a GPS enabled OS map app for your phone, it is only 100m or so into the forest which is not as dark as it first seems (the map location of the cairn is very accurate despite it being in thick forest).
As previously noted the ground is usually very damp, so appropriate footwear is essential.