The ancient Egyptians used an angular measuring system based on the horizontal offset from a vertical cubit rod. There were two different cubits; the common cubit of 6 palms and the royal cubit of 7 palms, the latter being used exclusively to measure pharaonic structures. A palm was further subdivided into 4 fingers, so a royal cubit was 28 fingers. Pyramid angles are therefore an integral number of 28ths. Khufu's for example is 22/28 or around 52 degrees.

Such a system is quite convenient for cultures that have not developed irrational fractional arithmetic (decimal points), so it is possible that the ancient Irish may have had some similar way of measuring angles.

21 degrees is approximated quite closely by a ratio of 3:8. Could there perhaps be some religious significance to these numbers? Three has always been a magical number. It's the smallest number of sides that defines an area; it's the minimum number of uprights to support a capstone; it's god, man and woman; knock three times; three times round a circle, etc. Eight is the number of fingers on both hands (excluding thumbs). By laying both hands side by side you map out a distance of 8 fingers. Then three fingers placed upright at the end defines the 21 degree angle. Pure conjecture, of course, but interesting to ponder.