Humewood Castle is closed presently for renovations and redevelopment (though the golf resort plans are presently being appealed by An Taisce), but luckily I bumped into some people at the gatehouse and was able to venture in for some pics.
The castle is a very large Gothic revival piece from the 19th century complete with walled gardens, lakes and a very large deer park. The co-ords on archaeology.ie place the stone in the middle of a disused tennis court but after some moseying around I spotted a large slab sitting incongruously between two paths very near the castle itself.
The carvings are well worn but immediately obvious once you are close, I doubt there is any time of day or year when the low sun would naturally bring them up at their present location.
A large three ring design with external gutter dominates the panel at about 14" across, surrounded by cups with single or double rings and occasional gutters. Simple cup marks are also dotted around and at the curved top end.
The gent I spoke to (who also assisted me in taking some pics by standing in the sun!) told me it may be moved on the advice of an archaeologist because of the risk of damage being on the verge of the road, it seems to bear fairly recent scars which are thankfully on the un-carved end.
The whole stone is about a metre and a half long, larger than it appeared in previous photos I had seen. Apparently it was found buried while one of the artifical lakes was being built in the castle grounds.