Can irritable bowel syndrome cause serious harm to intestines?

No. By definition, ibs is characterized by normal appearing bowel lining, but it's symptoms can be quite disabling. Diseased appearing bowel lining usually means a different diagnosis like ibd ( crohn's disease, ulcerative colitis, behcet's for example).
No. You may feel some serious pain and other GI complaints, but there's no proof of any physical harm to the intestines.