After looking at some of my pictures it does indeed (probably) face north of west, which is pretty unusual!

The one thing that we all need to consider (and I constantly get this thrown at me, by the way) is that the land between Proleek and Slieve Gullion would probably have been thickly wooded and it is unlikely that the hill would have been visible from the tomb.

Still, the alignment does look quite convincing. Nice find.