Presacral tumours represent a heterogeneous group of predominantly benign and occasionally malignant neoplasms. These tumours, though rare, frequently present either incidentally or with vague symptoms. Schwannomas of the presacral region are one variant described as benign tumours of neurogenic origin. The case of a large presacral schwannoma in a 26-year-old man, who was treated for irritable bowel syndrome for 4 years, is presented. The patient presented with intermittent constipation, a feeling of incomplete evacuation of the bowel and vague abdominal discomfort relieved by defecation. The symptomatology worsened and constipation became frequent, and the patient experienced increased urinary frequency. Baseline investigations were normal and ultrasonography of the abdomen revealed a pelvic mass, which on CT scanning was revealed to be a large retrorectal mass. The tumour was resected and histology revealed it to be a schwannoma. This unique case is presented to emphasise that irritable bowel syndrome must be a diagnosis of exclusion, especially if atypical symptoms are present.