DUBLIN (Reuters) - The lead European Union privacy regulator fined social media giant Meta 91 million euros ($101.5 million) on Friday for inadvertently storing some users' passwords without protection or encryption.

The inquiry was opened five years ago after Meta notified Ireland's Data Protection Commission (DPC) that it had stored some passwords in 'plaintext'. Meta publicly acknowledged the incident at the time and the DPC said the passwords were not made available to external parties.

"It is widely accepted that user passwords should not be stored in plaintext, considering the risks of abuse that arise from persons accessing such data," Irish DPC Deputy Commissioner Graham Doyle said in a statement.

The DPC is the lead EU regulator for most of the top U.S. internet firms due to the location of their EU operations in the country.

It has so far fined Meta a total of 2.5 billion euros for breaches under the bloc's General Data Protection Regulation's (GDPR), introduced in 2018, including a record 1.2 billion euro fine in 2023 that Meta is appealing.

($1 = 0.8966 euros)

(Reporting by Padraic Halpin, Editing by Kylie MacLellan)