Patsy Catsos, a dietician in Maine, wrote a book this year, entitled "IBS - Free at Last". She writes about the FODMAPs and how to conduct an elimination diet to determine if these are triggers for you. I followed it and found out that fructose and fructans, including wheat, are a trigger in large doses. However, lactose-free dairy doesn't seem to bother me.
When I faithfully followed the Eating for IBS diet several years ago, it took me awhile to determine that I could not handle oats or soy. Now I realize that apples in my daily morning of soy oatmeal was yet another trigger. I still haven't mastered it all - stress and fatigue play such critical roles, too. I am forever searching for balance. Good luck to you!