Suzanne Cleary and Peter Harding are a couple of dancers who've worked in the Riverdance stage shows. Recently they've begun to perform 'hand-dancing' on You Tube videos. The example above is one of their more intentionally 'po-faced' versions - in the America's Got Talent video, they actually smile a lot and move around the stage briefly. I'm not sure what they're doing on AGT, unless they were a guest act.
You can see the Riverdance origins in the kind of constant, unstoppable rhythm that pervades these videos. It's fun to watch two or three and then they become a bit much. Try some others another day!
Cleary and Harding have their own website, under their 'trade' name of Up and Over It. There are a variety of other videos on there, showing off different aspects of their dance style.