Is it normal to urinate 15 - 18 times within 24 hours with irritable bowel syndrome when the only symptom that day is fatigue? (i do not drink alcohol

Can be if anxious: It can happen, it's not normal nor dangerous, depends on Urine test results & how much water u drink & all ur meds !

Seems excessive: Not sure what it has to do with irritable bowel syndrome though? Perhaps you have been drinking a lot of water? If you are not having any pain with urination or fever, I would just see what happens. If it continues, it could be a sign of a bladder infection or other issue, even without other symptoms. Continued excessive urination for over 24 hours should be evaluated by a medical professional.

No: Frequency of urination can be due urinary tract infection or Diabetics or diabetic inspidus or urinary Tract obstruction. Need evaluation and UA