This gem is really easy to get to, if you disregard the fact that you have to cross a railway line. To reach the site, drive on the R294 from Boyle to Cloonloogh. Around 2.5km after you leave Boyle you come to a railway underpass, where the R295 bends right. Another 400m later you see a house on the right side, where you can easily park you car. Walk the farm track for about 100m and you reach two gates that help to safely cross the railway line. Be warned, as we visited the site, there were two trains that passed by.
This is really a magnificent portal tomb with a huge capstone, that sits in a threateningly angle on two massive portal stones. I'm not sure, if the capstone is in its original position, but there is now a supporting pillar, that stabilizes the whole structure. I only wonder why this site is not more mentioned in guidebooks of this area, because it really deserves more attention.
If you are in this area, I definitely recommend visiting this tomb!
I think this must be the right site for this story: (it needs to be near Boyle, Roscommon, and near the site of a mill near the 'issue of the river from the lake'). Please correct me if not.
At a short distance to the north of this mill, on the right hand side of the road going towards the lake, and not far off it, stands one the largest cromlechs that I have seen in Ireland. The sloping upper stone is fifteen feet long by eleven broad; its greatest thickness two feet six inches, and its average thickness might perhaps be safely set down at eighteen inches. It is now supported by four upright stones, but, once, had a fifth. To this, the neighbouring miller, in an evil hour, took a fancy, judging it would make an admirable stone for his mill; and with much difficulty and labour he removed it from its place; but just as the operation was on the point of being completed, the stone, to the amazement and terror of the bystanders, flew into a thousand pieces; an occurence which was interpreted as a judgement upon the miller for his audacious violation of this sacred work of antiquity. The people still look upon the cromlechs with a degree of respect, if not veneration, althought they have no notion of their origin, or of the purposes to which they were destined.
p278 in 'A Statistical Survey of the County of Roscommon' by Isaac Weld (1832). You can read it courtesy of Google Books, here.