The Health Service Executive has temporarily shut down its IT system following what it described as a “significant ransomware attack”.
The health body said it had taken the precaution of shutting down its systems to further protect them, and to allow it to assess the situation.
The issue has hit some services at Letterkenny University Hospital.
Outpatient appointments are going ahead and unless people have been advised otherwise they should attend.
However scheduled scans and x-rays are not going ahead and non-emergency elective surgery has being cancelled.
The hospital has confirmed that as well as x-rays, all CT, MRI and Ultra Sound scans have been cancelled today.
The Emergency Department is open but the public is being warned to expect delays and that people should only attend in the case of serious emergency.