St Vincent's Hospital in Dublin has said that 172 patients and 143 staff are now infected by the winter vomiting bug.
Due to the dispersal of the suspected cases throughout the hospital, practically all wards are now affected.
The hospital expects to cancel between 20 and 30 planned procedures this week due to the outbreak.
Patients are being advised to attend for their scheduled appointments, unless they have been notified otherwise by the hospital.
The A&E unit is under increased pressure because of the bug.
