Dublin's Tallaght University Hospital is cancelling all scheduled outpatient appointments and day procedures for adults and children on Wednesday.
It says power to a section of the hospital failed in the early hours of the morning.
The failure has effected a transformer, which means the hospital is running on generator power in certain areas - including radiology, outpatients, theatre and ICU.
"The hospital is cancelling all scheduled outpatient appointments, day procedures both for adults and children today.
"Please note this also includes radiology/x-ray appointments and phlebotomy/blood tests", it says in a statement.
It adds that dialysis, oncology and haematology appointments are not impacted by the power outage, so these should be attended as scheduled.
The CHI Children's Urgent Care Centre is also not impacted.
Appointments scheduled for Wednesday that are being cancelled will be re-scheduled as quickly as possible, the hospital says.
Affected patients will by contacted by the hospital directly.
The hospital is asking people to attend their GP in the first instance, where appropriate, instead of the Emergency Department.
