The monument is aligned to the rising sun of the equinox, its beam effectively halving its inside into a northern and southern part. At entering through the gate the circa 3 feet wide beam reaches the lower stones of the wall opposite. It seems very likely that the stone-lined path found at its discovery would have been of approximately the same width. Higher the morning sun rises shorter the beam becomes, moving slightly from south to north by circa 15°.
Since the so called drain is within such narrow proximity, I suspect that it may have been moved and the light could have touched either the small aperture or even ran through the full thickness of the wall to appear on the other side, which would not only have been a rather remarkable spectacle but also some stroke of ingenuity.
The direction of the beam also points towards the Seven Sisters, part of the Derryveagh Mountains, prominent visible from Grianán to the west. Perhaps it is no coincident that the star cluster of the Pleiades also carries the name of the Seven Sisters and I will need to come back at dusk to see where the sun is setting and if my non-existing knowledge of the night sky will allow me to detect the cluster, so evidently featured in ancient art – weather permitting.