Virgin Media Ireland said it will repay 100,000 of its customers a total of €3m in refunds after a ComReg investigation into "post cancellation charges".
The average refund amount due to customers is about €33.
The ComReg investigation found that Virgin was continuing to charge its customers after services were cancelled.
ComReg said the company failed to proactively refund customers and instead retained the monies in inactive accounts.
After its investigation, ComReg told Virgin Media that it intended to seek a High Court restraining order requiring Virgin Media to cease this practice.
Earlier this month, Virgin Media provided formal commitments to ComReg which will see over €3 million refunded to affected Virgin customers.
It said the refunds will be paid by the end of March.
In a statememt, Virgin Media said it takes regulatory compliance extremely seriously.
"It is our position that we are in full compliance with all our regulatory obligations," the company added.
It said it has "proactively co-operated" with ComReg throughout the entire process and noted that the investigation has now concluded.
Virgin Media also said it will review all other credits on any inactive accounts and refund those credits to customers by the end of June.
It also said it will not allow credit to remain on closed or inactive accounts in the future.
It has set up a dedicated helpline for customers to call in respect of the refund process and affected customers may contact Virgin Media via its helpline on 1800 941 770.
Information will also be available on its website for customers affected by the issue, it added.