The Office of the Data Protection Commissioner (ODPC) has said it will not investigate US firms for transferring personal data to a US spy agency because they have signed up to EU privacy principles.
The European headquarters of Apple and Facebook are based in Ireland.
The office had been challenged by an Austrian student activist group to investigate allegations that the US National Security Agency harvests emails and other private data from the companies in a mass electronic surveillance programme known as Prism.
But in an email published by the student group, europe-v-facebook, the ODPC said the companies were covered by Safe Harbour.
The Safe Harbour system allows US firms to certify themselves as compliant with EU data protection law by signing up to a set of principles supposed to safeguard how personal data are used.
The European Union adopted Safe Harbour in 2000, seven years before the NSA began the Prism programme that was revealed by fugitive intelligence contractor Edward Snowden last month.
"We do not consider that there are grounds for an investigation under the Irish Data Protection Acts given that 'Safe Harbour' requirements have been met," the ODPC wrote to europe-v-facebook.
An ODPC spokeswoman told Reuters: "If something is agreed by the European Commission for the purpose of providing safeguards, that ticks a box under our jurisdiction."
Max Schrems, the founder of europe-v-facebook, said: "We have the impression that the ODPC is trying to simply ignore the complaints and the whole Prism scandal."
The 25-year-old law student is also awaiting responses to complaints he has filed against Yahoo in Germany and Microsoft and Skype in Luxembourg.