I'm surprised that you mention the god Lugh but seem to place no emphasis upon the mythology surrounding Lugh and the folklore which has followed.
It seems reasonably clear to myself that the battle between Lugh and Balor corresponds to one of the planets which entered into a larger planet's plasmasphere resulting in electric discharges (the fiery spear of Lugh) and caused the "death" of that planet. I'll leave you to guess which ones.
Stonehenge, like Giza, is an astronomical marker, as you say, but with a specific purpose. And it wasn't just a sky/earth relationship per se. Death came from the sky. Shelter (Life) was under the earth. As these deadly planetary flybys which passed Earth were in circular orbit, these incursions into earth's plasmasphere could be determined with great accuracy. Hence the henge.
Post Edited (04-Dec-14 05:41)