The Office of the Ombudsman has said it is operating on the basis that data may have been taken in a cyber attack on its IT systems.
It said that systems have been taken offline to contain the threat of what it described as a financially motivated ransomware attack.
A forensic investigation is under way and the Data Protection Commissioner and An Garda Síochána have also been notified.
In order to protect people further, the Office of the Ombudsman said it has taken legal steps, including securing an injunction from the High Court to restrict any publication of potentially stolen information.
The Office of the Ombudsman provides IT support to other offices and as a result those services have also been disrupted.
The Ombudsman's ICT team is working with the National Cyber Security Centre and external cyber incident response specialists to assess the nature and extent of the incident and to support containment and recovery.
The Ombudsman, Ger Deering, said that the priority is to establish what has occurred, restore services safely and protect the people who rely on the services of the Ombudsman and the offices it supports.
"I want to apologise for the inconvenience or any concerns caused to the people who use our services.
"We appreciate your patience and assure you we are doing everything possible to resolve the matter and restore our services," Mr Deering said.
Updates will be published on the Ombudsman and relevant office websites as verified information becomes available.
The Office of the Ombudsman provides IT services to the Information Commissioner, the Commissioner for Environmental Information, the Protected Disclosures Commissioner, the Standards in Public Office Commission and the Commission for Public Service Appointments.
These services have also been impacted by the cybersecurity incident.
In an information note on its website, the Commission for Public Service Appointments said its telephone service has been affected, as has its ability to progress existing cases at this time.
"Due to the disruption to our systems, we are experiencing delays in processing complaints and ask that you bear with us," the Commission said.
"However, you do not have to resubmit your complaint or contact us at this stage," it added.
Additional reporting: Brian O'Donovan
