Patients attending the Emergency Department at Letterkenny University Hospital are advised to expect long delays as the radiology, laboratory and pharmacy systems are not up and running.
The Saolta Hospital Group says essential services like blood tests and diagnostic services are taking much longer to operate than usual as a result of the cyber attack.
Patients needing urgent care will be prioritised and people are asked to contact their GP or GP Out Of Hours Service in the first instance if their health problem is not urgent.
Where possible patients should bring their existing patient number or hospital number with them when they come to the hospital or any letter that they have received from the hospital as this will contain information which will make it easier to find existing medical records.
