About 530 million people had some personal information leaked, including, in some cases, phone numbers.
A digital privacy group is preparing to take a case to the Irish courts on behalf of EU citizens affected.
Facebook denies wrongdoing, saying the data was "scraped" from publicly available information on the site.
Antoin Ó Lachtnain, director of Digital Rights Ireland (DRI), warned other tech giants its move could be the beginning of a domino effect.
"This will be the first mass action of its kind but we're sure it won't be the last," he said.