The hospital says the ED is currently experiencing high demand due to more acutely ill patients and frail older people with complex needs.
There is also an increase in the number of Covid-19 cases at the hospital.
People are being advised to contact their GP or SouthDoc or to attend local injury clinics at St Mary's in Gurranabraher.
Local injury clinics are also open at Bantry General and Mallow General Hospitals.
The Irish Nurses and Midwives Organisation say there are 34 people waiting for a bed at the Mercy today.
Cork University Hospital is the most overcrowded hospital in the country with 60 patients on trolleys.
