“ We can see there's a large area of the wall that isn't faced with quartz, the several metres each side of the entrance, and the last few metres at the extreme ends. “

O ‘Kelly said “ we found that quartz was thickest and most extensive In the area outside the tomb entrance and at each side of it . “Which is also problematic for an accurate reconstruction . It also echoes what was found at Knowth and Knockroe , i.e. an emphasis of quartz at the entrance(s), the one area where the façade is quartz free .

“If the quartz was smashed it might suggest that much of it had already been broken up into smaller chunks for the wall facing, leaving some of them brittle? “

The smaller the chunks the less stable the (almost ) vertical structure is likely to be . The point is that they were "A good quantity …. and could not be used “ meaning that much of the actual material was dumped and had to be replaced .The replacements must have come from somewhere but not from the original revetment or sloping cover or platform .

This might be interesting .
http://www.meathheritage.com/index.php/event/hill-of-tara-lecture-series-2016-lecture-3