Celtic Thunder is an Irish singing group that has been performing since 1977. This a capella performance is from 2011. Listen out for the solo by their young bass singer about two minutes and 10 seconds in; a gloriously rich voice!
As always, try to listen on good speakers or a set of headphones.
By the way, I’ve just put a four-part arrangement of Amazing Grace on the Downloads page for members. You can look at the sheet music and listen to the rehearsal tracks for soprano, alto, baritone and bass here. There’s also a track of all four parts together.