Irish Dancing - Can someone help me understand it?

It's St. Patrick's Day - the traditional wearing of the green and yet another reason for Americans to drink themselves stupid (as if much encouragement was required).  I do not claim to be Irish and how could I?  Given that my genetic history is a mystery...who knows what lurks in that gene pool?  But that's a post for another  day.

I heard a story on the soon to be defunded (really, House of Representatives, really?  This is what you come up as a cost-cutter?) NPR this afternoon on my drive home.  Apparently, a flash mob featuring Irish dancers erupted in the Sydney, Australia Rail Station today.  Turns out it was stunt by the Irish Tourism Board.  As anything Australian always captures my attention, I had to find the video when I got home.  Here it is:

I just don't get it.  And by it, I mean Irish Dancing.  Why no upper body movement?  When I see this dancing, I can't help but think the lower half of the dancers' bodies are in some kind of serious seizure and the upper half of their bodies are locked down in straight jackets.  Why so stiff?  Again, I just don't get it.  Can someone help me understand it?  Do I need to?  Probably not.  Happy St. Patrick's Day anyway.