Ireland’s health care system was hit by a major ransomware attack on Friday, forcing its health service to shut down its IT systems and locking many hospitals out of their computers, in what one government minister said was possibly the most serious cyber attack in the country's history.

The ransomware attack began overnight, targeting Ireland's Health Service Executive which said it had decided to shut down most of its IT systems as a precaution.

Many hospitals and clinics reported on Friday they had lost access to their computer systems -- suddenly shut out of patients' records, appointment booking and email systems -- prompting some to cancel most non-urgent appointments. Those facilities said they had contingency plans in place; medical equipment was not impacted; and care was being given as normal to patients.

Read more: ABC News