The Accident & Emergency Department at the Mater Misericordiae Hospital in Dublin is closed due to flooding.
The hospital is asking people to attend other hospitals for A&E treatment.
A roof in the St Camillus ward in the old part of the hospital began leaking overnight. No-one was injured but patients had to be moved to different wards as a precaution.
About 25 patients had to be moved from the affected areas.
The Mater Hospital said work on a temporary emergency department will begin tonight, but warned members of the public not to attend until further notice.
"Initial safety reports show extensive flood damage to A&E and two ward areas," said a spokesman.
"The two ward areas affected will require extensive repair works over the coming weeks.
"Patients scheduled for elective surgery will be contacted by the hospital to confirm if their scheduled surgery has been affected.
"Outpatients' services are not affected."