The vomiting virus at Dublin's Beaumont Hospital showed no signs of abating yesterday, with 24 confirmed cases and 74 suspected cases reported.
All elective and non-emergency admissions have been cancelled.
The teaching hospital's accident and emergency department remained open but was operating at a reduced capacity.
All ambulances with emergency cases from its catchment area were being diverted to the Mater Hospital.
The Eastern Regional Health Authority has asked the Mater to postpone booked admissions in recognition of the added pressures which it faces through its A&E department.
The Norwalk-like virus (NLV) has spread to 16 of Beaumont's 19 wards, making it difficult to isolate non-infected from infected patients.
A number of nurses and other staff members have also been affected by the bug.
The outbreak of gastroenteritis first affected the hospital two weeks ago and should begin to wane this week. However, it took six weeks to completely eradicate the virus from Cork Regional Hospital following an outbreak earlier this year.
A spokesman for the hospital urged the public not to visit Beaumont unless absolutely necessary, and said that children would not be admitted under any circumstances.