This "syndrome" has led me to test many extremes. Everything I say is the result of my own experience and we all know--or should know--that there is huge individual variation. So, the idea that "this is good for IBS and that is bad" just doesn't work.
By a kind of accident, I discovered that most carbohydrates--and especially fibrous and raw ones--tend to exacerbate my condition.
One day, many years later and in total desperation, I wondered what would happen if I ate no carbohydrates, as I had read Viljhamur Stefansson had done. So I started surfing and there was a group of "zero-carbers" who ate only meat. And I gave it a try for 7 months.
So, I was eating lots of protein and some fat. I bought ribeye beef in bulk, which is one of the fattier cuts.
I had diarrhea the entire time. Everyone was encouraging me to stick it out, but I eventually decided that it wasn't going to get better.
In the book Life Without Bread the authors state that Chilean authorities would kill the political prisoners by giving them only lean meat. I don't know if that is true. But I now wonder if zero carb might not be so bad if it were implemented not in a meat-only form but merely a zero-carb form. That is, allow all zero-carb foods, which basically means meat (including fish) and fat. I haven't tested this but I have been adding fat to my meals for a few years and it has definitely helped. I track my nutrition on an app (Cron-o-meter) and the significant increase in fat has led to a decrease in protein.
Now, sufficient protein is essential to life. So, I am not advocating that anyone cut out or drastically lower their protein consumption. But in my case moderate--as opposed to high--protein consumption, accompanied by intentionally adding fat, along with extreme carb restriction (but not zero carb) has ameliorated my condition, though I still have a long way to go before I would call myself cured.
When I first started this a few years ago I was very uncomfortable adding fat to my meals. I typically use lots of olive oil and (grass-fed Kerrygold) butter. But now when I have to eat meals that I have not prepared, their lack of sufficient fat makes them less palatable. At restaurants I try to hold onto the olive oil after the salad is removed and I horde the little pats of butter, if I can.