I don't think Angel is irredemable. I didn’t realize until lately that I’m in the minority of my view on Angel: that he’s a fascinating character precisely because his sin is hubris and because he constantly makes decisions for other people’s “good”—decisions which I can’t in good conscience condone (see…most of AtS and half of BtVS, actually). So to me, having Angel come riding in on his white horse to save Buffy from having to tarnish herself or get blood on her hands by doing something he believes to be “necessary” (instead of recognizing that the thing that makes Buffy Summers so awesome is that, at the last minute, she always finds a way to get around what’s necessary but not good) makes absolutely perfect sense. I see it as completely compatible with his earlier characterization, especially Not Fade Away, which I believe to have presented Angel in a completely Machiavellian light (poor Drogyn! I cannot separate Alec Newman from Paul Atreides, and I’m always: OMG! ANGEL KILLED MUAD’DIB!). Does that make him irredemable? No. But it does say, to me at least, that he believes that that is his role, and that's the one he's going to keep fulfilling: the one who does things others can't or won't do (regardless of whether those things are morally wrong). And I think that's anything but noble, though clearly others would disagree.
Of course, I’m also one of those people who think if he ever actually got the Shanshu, he’d be totally and completely miserable, and that his real reward is Connor, and he should recognize that.
So clearly, my views are a little unorthodox.
Lirazel, i think there's a small difference because Batman didn't kill those people. He becomes whatever the public needs him to be, because he is strong enough to handle it. His reputation/image is tarnished but his hands aren't. With Angel, those killings are on his hands, directly or indirectly he is partly/fully responsible.