Heavy rain has caused flooding in the grounds of Letterkenny General Hospital and the emergency department has gone off call until midnight.
The fire service is at the scene, close to the emergency department, pumping water away.
Patients already in the emergency department and hospital inpatients have not been affected.
Members of the public are asked to attend their GP, Now-Doc, Sligo Regional Hospital or Altnagelvin.
Full emergency department services are expected to be restored after midnight.
The hospital said in a statement: "After torrential rain in the Letterkenny area earlier this evening, there has been some flooding at Letterkenny General Hospital.
"As the car park and roadway are flooded, access to the emergency department is very curtailed.
"There is a low level of flooding due to water leaking into the reception and administrative areas at the emergency department but clinical areas have not been affected."
Sandbags were put in place at the door of the department, which was refurbished recently after flooding damage last year.
Severe flooding last year damaged 40% of the hospital and forced the closures of many departments.
In March of this year, the new emergency department was officially opened by then minister for health James Reilly.
It is located in a new medical block, which cost €24m.
Hse says clinical areas NOT affected by flood at #letterkenny hospital - low level flood in reception and admin— EileenMagnier (@EileenMagnier) August 5, 2014