The Health Service Executive has suffered a "significant ransomware attack", prompting it to temporarily shut down its IT system.
Here is what we know so far:
Overnight the HSE became aware of what its CEO Paul Reid described as a "significant" and "sophisticated" ransomware attack.
The attack has impacted all of its national and local systems, which are involved in all core services.
HSE Chief Operations Officer Anne O'Connor said the HSE was alerted to the attack at 4am and she described it as a "zero-day threat with a brand new variant of the Conti ransomware".
The HSE has said a ransom has been sought, and will not be paid, in line with state policy.
It is working to rectify the issue through the weekend.
What is ransomware?
Ransomware involves an attacker trying to gain control over the data held by an organisation or individual. This can be done through software that blocks access to files, or by the attackers stealing the files from a system.
They then demand money for its safe return.
How has the HSE responded?
The HSE says it took the decision to shut down its IT system, in order to protect it from the attack and to give it time to assess the situation.
It says it is working to find out the extent of the attack which may take a number a days, and is working with the National Cyber Security Centre (NCSC), An Garda Síochána, external and international partners, and its security partners to see what steps it should take next.
Ms O'Connor says all systems were shut down as a four-step approach was initiated to inform people about what had occurred.
The HSE is not aware that any personal information has been compromised, she said.
Ms O'Connor said the HSE's system virus protections are updated on a daily basis.
She said: "We don't know where it came in or which systems are impacted" so the precautionary step to shut all systems down was taken.
What does it mean for patients?
Health service providers around the country have been left to assess the impact while continuing to care for patients.
Many hospitals have described situations of being compromised without access to their electronic systems and records, and having to rely on contingency measures such as paper-based systems.
In some cases, hospitals have warned of significant disruption with routine appointments cancelled today, including maternity check-ups and scans.
With delays expected to some patient services, health providers are also warning this IT outage could pose difficulties in accessing patient results.
Ms O'Connor said that some stroke and cancer care services have been impacted and that "the situation will be very serious if it continues into Monday".
She said the big concern is in relation to diagnostics, with radiology systems gone down.
This is preventing CT and other scans from going ahead.
She said that will impact people with stroke care and early diagnoses of stroke affected.
For example, St Luke's hospital in Dublin has cancelled all radiation oncology, with support for urgent cases put in place.
Maternity hospital systems have also been impacted.
Assistant Secretary General at the Department of An Taoiseach Liz Canavan said that all those with appointments today and in the coming days for any HSE service should attend, unless asked not to.
Emergency departments are operating as normal and any concerns about appointments should be made to local hospitals.
Mr Reid said the attack and IT shut-down has not had an impact on equipment in hospitals, which are run separately to its data system.
Dublin's Rotunda Hospital was the first to announce the cancellation of most of its appointments for the day due to the IT issue.
It said that only those who are 36 weeks' pregnant or more, or those with urgent concerns, should attend.
Are Covid tests or the vaccine roll-out impacted?
Tánaiste Leo Varadkar said the Covid testing programme was affected.
The GP and close contact testing referrals system is down. GPs will advise symptomatic patients to attend any walk-in sites if they need a test.
But the HSE says scheduled Covid-19 tests will go ahead as planned and advised people to attend for a test if they have an appointment.
The HSE says it will focus on delivering positive test results and advised all those awaiting results that they may need to isolate for longer until their result comes through.
Contact tracing is continuing out of two centres.
Anyone who is concerned they might have Covid-19 is asked to contact their GP.
The HSE says the cyber attack has not impacted the HSE's vaccination programme.
What about testing and tracing?
People who had a recent Covid-19 swabbing appointment and are awaiting their results will still have their tests processed in laboratories.
There may be some delay in receiving test results, the public is asked to be patient while a new process to provide results, with an initial focus on detected results, is implemented.
Anyone who is awaiting a Covid-19 test result must self-isolate until they receive their test result.
This is a change from the usual restricting movements advice.
If you have an appointment today for a swab test you are asked to attend the designated test centre and have the test.
Contact tracing has been moved to two sites that remain operational. Contact tracers will call with detected results and will gather the required data.
Importantly they will gather close contact information and call close contacts to ask them to attend a walk-in site for testing.
This process may take longer than usual.
What happens if my GP needs to refer me for a test?
If you have symptoms, or are a close contact of a positive Covid-19 case, the normal method by which GPs refer patients who require a test, either because they are symptomatic or they are a close contact, is unavailable due to the IT shut down.
This means that GPs cannot currently refer new people for Covid-19 testing at one of the HSE testing sites.
GPs will advise a patient to attend a walk-in facility if a test is required.
Walk-in testing will be prioritised for symptomatic people and close contacts of confirmed Covid-19 cases.
From this afternoon, anyone who feels they might have a symptom of Covid-19 can attend any of the static testing sites or walk-in test centres and they will be able to get a swab test without a GP referral.
What about Tusla?
Liz Canavan said that IT systems at Tusla are not operating, including email and the portal through which child protection referrals are made.
She said that this is for security reasons as the agency is hosted on the HSE network.
Ms Canavan said that a person wishing to make a referral about a child, can do so by contacting the local Tusla office in their area.
What else is impacted?
The HSE's Anne O'Connor said the shutdown was particularly impacting acute hospitals and corporate functions, with email down.
HSE payments, payroll and medical card eligibility services have also been impacted.
Steps to continue to assist medical card holders and those using the Fair Deal Scheme are being taken.
Ms O'Connor said that virtual appointments can continue today, but not any appointments that are via Zoom or Teams
Who is behind the attack?
Paul Reid described the attack as "sophisticated" and "human-led" and said it was a major incident for the executive.
The Minister of State for Public Procurement and eGovernment Ossian Smyth said it was an international attack, by a cyber criminal gang, looking for money.
"What they're attempting to do is to encrypt and lock away our data, and then to try to ransom it back to us for money."
He described it as "possibly the most significant cyber crime attack on the Irish State", and said the Government is "deploying everything" in response to this.
Minister Smyth said this is "the one that got through" - saying that there is a "constant bombardment" on State data.
When will it be dealt with?
HSE Anne O'Connor warned it is unlikely to be resolved quickly and could continue for a number of days.
How long it takes to resolve depends largely on the nature of the attack, according to cyber security expert Brian Honan.
If the IT system had been compromised it could be restored from a back-up quite quickly, however it was possible that the attackers had gained access to that too.
If that was the case the HSE could try to rebuild its system from scratch, or it could choose to pay the ransom demanded. However he cautioned against that approach, pointing out that the people involved would not be the most trustworthy.
When will we know more?
Mr Reid said that regular communications will be forthcoming during the day as the situation evolves.