Hi HB
I suffer from IBD(Inflammed Bowel Disease[Crohns]) and not IBS(Irritable Bowel Syndrome).
I fly professionally(shorthaul) and hold a Class 1 medical. Not sure about IBS, but when I have a "flare up" of IBD I am grounded for some considerable time.
Crohns is a chronic disease and therefore I am on drug therapy for the remainder of my life, unless there is a major medical break through and a cure is found!
One restriction I have found is that you are a captive within your present airline-if they are tolerable of all the down time-and any future employer will not look favourably on a new employee who has a known disease.