Forgot to mention that at places like Loughcrew the tombs face a multitude of directions which some people say provide a whole calendar.

At Knowth all the smaller orbital tombs face inwards towars the centre of the main mound (which appears to have been built after the smaller ones)

A similar thing happens at Carrowmore where all the tombs point at the central (and much later cairn) known as Listoghil. Beneath this cairn is a very ancient clay platform that seems to hhave been a pyre platform and the focus of Carrowmore - the oldest known passage tomb cemetery (7500-5500bce or so). Even more insane is the fact that the passage tombs on the surrounding mountains also seem to point at the same spot.