I've been reading O'Kellys superb book on Newgrange and remembered this thread when I came to the bit about the door stone. The outward face as seen now is not carved or worked in any way, the other side acted as a paving stone outside the entrance after it fell and was worn completely smooth from all the feet passing over it. The un-worn face was placed outwards so it wouldn't give the impression it had been deliberatly smoothened by the builders...

The book also mentioned when they tried placing it over the passage entrance it fitted the gap perfectly completely sealing the entrance.