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

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