I got to see that in person! Newgrange is fascinating.
I know folks here are arguing about Celtic pagan culture but the spirals actually come from neolithic peoples from 5,000 years ago that we know very little about. They predate the Celts and the Picts and even the pyramid builders of ancient Egypt. We don't know their language, religion, or much of anything except that they were pretty good engineers, moved enormous stones tens or hundreds of kilometers, and had a thing for spiral motifs!