It is the largest fine in the authority’s history, but Instagram’s parent company Meta said in a statement it planned to appeal.
Ireland’s Data Protection Commission (DPC) began an inquiry in September 2020 in relation to how the social media giant processed the details of teenage minors on Instagram accounts.
The probe looked into a process in which users aged between 13 and 17 were allowed to operate business accounts on Instagram, which in some cases allowed, or required, children’s phone numbers or email addresses to be made public.
It also examined a process in which Instagram’s registration system resulted in children’s accounts being set to “public†by default, which made the content uploaded by child users public. However, this could be switched manually to private.
A DPC spokesman told the PA news agency: "We adopted our final decision last Friday and it does contain a fine of 405 million euro."
The social media giant will now reportedly be given a week to respond to the DPC’s decision.
A Meta spokeswoman said: “This inquiry focused on old settings that we updated over a year ago, and we’ve since released many new features to help keep teens safe and their information private.
"Anyone under 18 automatically has their account set to private when they join Instagram, so only people they know can see what they post, and adults can’t message teens who don’t follow them.
"While we’ve engaged fully with the DPC throughout their inquiry, we disagree with how this fine was calculated and intend to appeal it.
"We’re continuing to carefully review the rest of the decision."
In September 2021, Facebook-owned messaging service WhatsApp was hit with a fine of 225 million euro (£194m) by the DPC for breaching European Union laws on transparency, and the sharing of user information with other companies owned by Facebook.
This was the largest fine issued by the DPC before the Instagram fine issued on Monday and was also appealed against.