I must admit that the first time I heard it I was wondering as well. But I have listened a few times and I must admit I am now not sure that he is off key. I believe it is harmony but not an octave away. I think he is singing the fifth. A G to Ronnie Drew's C so to speak. Not unusual at all but Ronnie's voice is so intricate by it self anything and anyone may sound odd, harmony wise.

When I think of great male voices in Irish music, Luke Kelly's is certainly to the best that ever was in my life time. If there is a close second it was Joe Burke's. There are lots of other great voices to be sure, among them the much maligned Ronnie Drew, but Luke was just beyond them all.

I would like to hear a duet with Ronnie Drew and Tom Waits some time. I think that would a harmony made in heaven. Certainly a back room in heaven but heaven just the same.

Don