It's understood the incident at the Emergency Department was a small electrical fire that was quickly brought under control.
Part of the hospital's Emergency Department was evacuated after the smoke alarm was activated.
There are no reports of any injuries.
Fire Control told RedFM News that three fire tenders and a hydraulic platform attended the incident after the alarm was raised at around 1:10pm.
In a statement to RedFM News, CUH say a number of ambulances were diverted to the Mercy University Hospital and the affected area was declared sate for patient care at 2:30pm.
The hospital have thanked their staff, the National Ambulance Service and Cork Fire Brigade for their quick response.
