guinness commercial

OK, I really know nothing about music, so this may be a stupid question…

In that Guinness commercial on TV, where people on a train are singing, what is the song? Hopefully, someone out there is familiar with the ad I mean. It’s definitely something I’ve heard before, but like I said, I don’t know squat about music.

You’re not the first person to wonder about this.

http://boards.straightdope.com/sdmb/showthread.php?threadid=39055

One assumes you are referring to thisthis

If so, it was Carl Orff, O Fortuna.