This site can help you find out what fiber is good for IBS http://www.helpforibs.com
Well you can actually manage IBS with your diet so read some websites on "IBS diet" and you will learn a lot.
Also constipation can be relieved by fibre yes, but better yet if that doesn't work is take magnesium (citrate or chelate) with meals and calcium. Calcium and magnesium go together as calcium has a relaxing yet constipating factor and magnesium has a relaxing effect that gets the bowels moving wonderfully. Try 250-1000 mg to start 3 x day and see if you need more gradually. When bowel movements become frequent then slow the dosage down as everyone is different.
Good Luck
You might get some special diet that is high in fiber, and might have to take supplements, like Metamucil. BUt there are something that they can do. Just go see your Gastroenterologist and he'll try different methods. Eventually it will work.
Good Luck