It's called A Bright Star In Cepheus, and I marvel at how these two played it in perfect unison, as though a conductor was in front of them waving a baton. They varied the pace as they wished, slowing for effect, then sweeping into the next passage.
There were little subtleties too, as in where Phil Cunningham stopped using his right hand, just played the broad backing chords allowing Aly Bain to take the "lead" as it were. It obviously still means a lot to Cunningham, as Aly Bain felt the need to offer a hand after the final chord.
The piece isn't that old, though it sounds like it's been been around forever. Johnny Cunningham died in 2003 of a heart attack, he was 46.
Here's the two brothers in another clip. If you like your Celtic music fast, just look and listen to the virtuosity of these two.
http://www.youtube.com/watch?v=fEGxWWctBtw&feature=related