I'll have to disagree on the above interpretations, but then again, we might all be wrong.
Why would Ennis say "I'm sorry"? Why would he say anything? He grunts, once or twice, but this is new territory for him - he's rigid, slow to move, his mouth clamped tight.
I think Jack is saying [it]saright, sallright, it's all right. Admittedly it sounds like sorry, but I don't think it is.
The script (screenplay, which *may* differ from final dialogue) offers no words from Ennis, only Jack saying "It's all right....it's all right."
There are clearly three utterances in the film, so debate is open. Just my opinion.
btb