If you are eating bananas OTH half should be all and do not eat them when they are ripe.
The problem the research at Monash University (Australia) have found it that many with IBS suffer a malabsorption of short chain carbohydrates (FODMAPs). This includes many of the natural sugars.
The major FODMAPs are fructose (honey, apples, pears…), fructans (wheat, rye, onions…), GOS (legumes, cabbage…), polyols (sorbitol and mannitol in stone fruits, mushrooms…) and lactose (milk, ice cream…).
There are lists on the internet but it is better to ask your doctor if you can do this through a dietician as you may well be able to add some items back in - and should if you can.