Other theories are available
My favourite theory references that not only do Irish dancers keep their hands by their sides, but they also aim for a very straight and non-mobile upper body. Apparently this stems from more puritanical times when dancing was frowned upon if not prohibited.
When dancing only, the legs/feet move. Which means that you can dance in the pub and the authority figure wandering by and looking in the window only sees the upper bodies, which aren't moving
Biting the hand that feeds IT
