Radnorlad wrote:
Would they cut trees down & lie the branches out in line with barrows? Or just build with their branches?
There is an excellent book by Ronald Hutton on the druids called 'Mistletoe and Blood', a review here......
http://www.history.ac.uk/reviews/review/846
It will explain the history of druidism but not answer how the barrows were aligned, could be they were following a trackway ;)