I think it should help for IBS-C, but really only if you need more good-bacteria. Only way to really know this is to try and see if it helps. I did find it helps get more out at once, which I would assume is great for IBS-C. Never thought I would be happy as IBS-D to have bigger BMs but they feel great!
As far as I know I had IBS since I was a kid. I have no idea if anything 'triggered' it to start since I can't really remember as a kid if I had a lot of antibiotics for things like ear infections. I didn't have any major surgeries that brought on symptoms either. All I know is that every time I've had to take antibiotics it's been awful. My stomach would cramp up all day and I'd be sick constantly. It would normally get better after being done with the antibiotics, but never 'completely' better. And every round of antibiotics ended up being worse than the last.