Ach. I used the wrong term - looked it up in the dictionary at home. I should have said ledges or shelves. I've posted some pictures of both circles anyway, to hopefully show what I was on about.

Your cairn actually isn't far off date-wise. The nearest extreme of the range for Drombeg and Cashelkeelty is 794BC, Reenascreena a bit further back and thats the only info available. There's been just a few excavations over the years, the most recent being, I think, William O'Brien's at Lissyvigeen. Lettergorman's construction date could, I'd imagine, be anywhere within a long swing of 1000BC.

[I checked the bearing between the circle centre and the point that the quartz boulder seems to direct you to; 187.189 degrees, 1.129km distant, though obviously the stone is so wide and close that I could be way off in my assumption.]

Do the one cup rocks (thanks for the pics!) have a trend for facing in any particular direction, either in the way the slope faces or, perhaps, the way the rock is positioned?