Vodafone is to refund nearly 73,000 customers a total of €416,972 after the regulator found it had not complied properly with customer information requirements.
Under the law, Vodafone is required to provide users with a contract on a "durable medium" setting out the terms.
But following an investigation, communications regulator Comreg found the telecoms company did not comply with these requirements between September 2016 and March of last year for customers of the operator's "Extra" Pay as You Go mobile phone offers
This led to a situation where customers who topped up their accounts by the monthly tariff price of €20 or €30 during the four week offer period were unaware that it would lead to their accounts being reset.
This meant that they lost any remaining allowances as they would not be carried forward and a new cycle would begin.
Comreg has accepted a written commitment from Vodafone that it will take a number of steps to address the issue.
This includes refunding 72,774 customers that may, in the past, not have intended to reset their offer when they topped up.
According to Comreg, the total amount to be refunded is €416,971.79.
Any of the total refund amount not paid to customers will go to Comreg.
From now on, Vodafone will also advise customers in their contract at the point of sale that their offer will reset if they top up by the amount of the offer.
The company will also amend the text message sent to customers that top up to clearly tell them that their offer has been reset.
Provided Vodafone complies with the commitments, Comreg will not take legal action against it.
In a statement, Vodafone said it worked closely with the regulator on the matter.
"Following discussions with Comreg, Vodafone revised certain offer communications and have agreed to apply credits to those customers impacted," it said in a statement.