Three Ireland is to refund around 14,000 customers approximately €3.76m in roaming charges.
It follows an investigation by the Communications Regulator ComReg.
The inquiry looked at Three Ireland’s compliance with EU roaming regulations, specifically relating to the messages that consumers receive while roaming.
When travelling in the EU, customers can use their mobile services like at home, but providers may have fair-usage policies on data allowance limits.
Upon arrival, customers should receive text messages or alerts about the data allowance and again if that allowance is used up.
ComReg found that in the case of Three Ireland messages were not being sent or did not contain the required information.
“Three Ireland will ensure that all consumers receive relevant messages at appropriate times, containing necessary information,” ComReg said in a statement.
“As well as agreeing to refund affected customers, Three Ireland will ensure that the notification issued to customers when they reach both the €50 and the €100 default roaming limits per monthly billing period offers information regarding roaming add-ons which can reduce roaming charges and shall include the per unit cost associated with each additional unit consumed whilst roaming,” it added.