The Emergency Department at St Luke's General Hospital in Kilkenny has been evacuated due to a fault in the ventilation system.
The unit is expected to re-open early this afternoon.
Patients are being treated at the Medical Assessment Unit and the Stroke Therapy area in the hospital.
The Health Service Executive said acute emergencies would be prioritised.
However, people are being asked to attend their GP first for non-emergencies instead of going to the Emergency Department.
St Luke’s has apologised for any inconvenience but said normal services elsewhere in the hospital are unaffected.