The one on Do or Die by the Dropkick Murphys, who is the second singer? I know one is Al Barr, but who is the other?
No, the lead singer on that CD isn't Al Barr, it's Mike McColgan.

The other guy is Ken Casey I think. I'm pretty sure he does most of the backup vocals.
I don't think I meant Al Barr, I meant the one that does Street Dogs. I believe I may have gotten them confused. But that does not sound like Ken at all. Not a single bit.