This "cairn" is just off the N8 at Johnstown on the main Dublin-Cork road. Recently Ive been passing this way quite a bit and wondered about the funny looking trees on top of the hill (They are pretty distinctive from the road).
I checked the OS map and it looked to me as the trees corresponded to the position of a cairn so I decided to take a look.
Head into Johnstown and turn left and then left again where there is a sign for the Spahill. It should be easy to find from here.
I'm not sure about the cairn itself. It seems to be a tree covered mound. The mound itself isnt very distinctive. However the fact that a wall has been built around it to protect it says to me that it is of some importance locally.
It may be possible that the wall around it was built from some of the material from the cairn. I dont know.
There is also a funny feature in the wall. A little "portal" no more than half a metre high. Maybe its for letting wild animals (badgers?) in and out of the enclosure.
I imagined it was left for the little people to get back into their Sidh!