At lock, Donnacha Ryan can step in, but he's just getting back into form (like Healy). Dan Tuohy might get called up from Ulster.
As far as back-row ballast goes, I believe O'Mahony is going to be okay, but if not, it'll most likely Chris Henry. Jordi Murphy can jump in, but I think he'd need to up his intensity a bit to match what's happening on the pitch now (completely possible). And if need be, if Tuohy comes in and Ryan starts at lock, Henderson can also cover blindside flanker.
It's a damn shame Tommy O'Donnell dislocated his hip in the warm-up game against Wales, because he had some form. That was a six-week injury back in early August, but I doubt he'd walk right back in. Rhys Ruddock was another possibility -- he had some shining moments in the 6N -- but he broke his arm in June for the second time, and I believe went off injured in a Leinster game last week (but that may have been precautionary). CJ Stander qualifies to play for Ireland the day after the RWC final, so he's out.
Overall, I think they'll be alright, although they'll miss O'Connell's generalship around the park. But I also think that'll kick them on to perform even better.