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?
Any idea of the difference in height between the circle and the horizon
if it's relatively flat a solar /lunar alignment seems unlikely but a difference of 250 m in height would possibly provide a solstice alignment and 160m a lunar . The rocks all have different orienations some on slopes but not all although the surface is always flat and the cup usually found away from the slope .
