Criminal hackers gained access to the HSE IT system some time before the cyberattack was noticed, Communications Minister Eamon Ryan has confirmed.
Mr Ryan told the Dáil it is "likely" that the hackers got into the system before last Friday as such attacks are prepared in great detail long in advance and often require significant periods of time for the criminals to gain access and gather information before alerting authorities.
Specialist IT teams are now working "around the clock" to assess 80,000 HSE devices that have been impacted by the cyberattack.
Health Minister Stephen Donnelly said it will take "several weeks" and "huge efforts" to get the 2,000 servers and the 80,000 devices back up and running.
Sinn Féin's health spokesperson David Cullinane suggested that the Government had "taken their eye off the ball" in relation to cybersecurity and that more funding should have been allocated to protect vital IT infrastructure long before now.
Mr Ryan said that 29 people are currently employed in the National Cyber Security Centre, but he added that these staff are part of a far wider team of experts who work across Government departments.
Mr Donnelly highlighted the crippling impact of the attack on health services, but said the HSE is prioritising those patients who are most in need of urgent care.
"Unfortunately, radiotherapy services are particularly impacted," said Mr Donnelly. "Medical oncology is continuing — with some delay due to manual recording and administrative processing.
"Plans are being developed to relocate some services to private hospitals."
Mr Donnelly added that a significant number of outpatient appointments have been cancelled, especially in dental, orthodontics, ophthalmology, and audiology services.