The carving of stones on the 'unseen' face is a feature that occurs in some North Yorkshire barrows.
The excavation of Howe Hill near Brotton, revealed 24 cup marked portable stones all placed with their carvings facing into the barrow. There have also been recorded excavations that have revealed carvings on the unseen side of capstones.
Art for the after life?

I'm not really sure how easily they give the key out by I'll tell you where to get it from. You need to speak to Claire Tuffy at the Newgrange Visitors center and give her a 65 euro deposit.

I got the impression that they don't hand it out to everyone and that megalithomania helped a little.

As for the carved stones.

The one at the rear of Dowth (with the seven sun motifs on it) has some very good stuff on the back and this actually sparked off searches for a third tunnel. Three tunnel passage tombs do exist - Baltinglass Hill is an example and The (now famous) Druids' Judgement Seat was one.

Many inaccessible and invisible faces are carved (a lot at Knowth) and it is assumed that perhaps they are to give the stones some power.

I put forward another little theory. Hows about they are actually from an older tomb and have been recycled?