I just leafed through one of Gimbutas' books I have for photo comparison. The spirals she includes closest to the ones I see in Post 13, though not identical, are some 5th-6th millennia designs from Eastern Europe, related to snake goddess imagery. I didn't see anything with the distinctive tri-spiral, though, which is an interesting feature.
The best books on decoding prehistoric symbolism I have read are Gimbutas' Goddesses and Gods of Old Europe and Adrian Bailey's Caves of the Sun. I would tend to interpret the Newgrange stuff along Bailey's lines. He compares the structure of a number of megaliths and myths with ancient beliefs about parallels between the annual seasonal cycle and the birth-death cycle, arguing that dolmens and similar structures are designed to draw a symbolic parallel between death and the decline of the Sun during winter (to sum up a detailed argument without doing justice to the book).
And as a Tolkien fan I can't help being reminded of Helm's Deep when I read this part of the first post :-)
It would be a brave man that would come down one of these after you - not knowing the plan of it and not knowing at which corner he stuck his head round you'd be waiting on the other side with an axe or whatever.