The observations of the team that excavated Newgrange lead them to believe that sighting rods external to the site weren't necessary. Once the location had been observed it would have been enough to mark the chamber centre and the passage entrance point.

For a really good book on Newgrange see O'Kelly - Newgrange. I can dig out the ISBN etc if anyone wants it. O'kelly is the bloke that was in charge of the 15 year project there. It's a 'real' view of the monument from observation and admiration of the builders, without loads of New Age speculation to clutter it up. A truly great book - in my mind the *only* book I've read so far on the place fully worth the paper it's printed on.