Ireland, in its role as the EU's lead data watchdog, said Tuesday it had fined Twitter 450,000 euros after the social media platform breached privacy rules.

The fine, equivalent to $539,000, follows the first cross-border case against a tech giant under GDPR, the landmark EU data protection charter launched two years ago.

Ireland hosts the regional headquarters of Twitter -- as well as Apple, Facebook and Google -- and is therefore largely responsible for policing their European activities. 

"The Data Protection Commission has today announced a conclusion to a GDPR investigation it conducted into Twitter International Company," Tuesday's statement said. 

"The DPC's investigation commenced in January 2019 following receipt of a breach notification from Twitter."

Ireland looked at whether Twitter had informed its data protection authority of a breach within 72 hours and properly documented the event.

Twitter could have been fined up to four percent of its annual global turnover -- a $140 million wedge of the firm's reported $3.5 billion 2019 revenue.

The far smaller penalty handed down is "an effective, proportionate and dissuasive measure" after Twitter "infringed... GDPR in terms of a failure to notify the breach on time to the DPC and a failure to adequately document the breach", the statement added.