Around 990,000 householders are eligible for a refund.
Irish Water has warned it's "inevitable" that some water charges refunds will be delayed, particularly among those who have moved houses since paying the utility - writes dublinlive.ie.
The utility made the claim ahead of upcoming legislation which will see domestic water charges refunded to homeowners who paid them following their introduction in 2015.
And it warned the 990,000 householders eligible for a refund to make sure the address they have registered with the utility is correct - or they will face delays in getting their payment.
The company recently revealed that it will refund customers who paid the charges by sending a cheque for the total amount paid to houses across the country.
But it warned that those who have changed address since making a payment may risk having their cheques sent to the wrong address if they don't update their details.
Speaking ahead of the anticipated move, Head of Customer Operations at Irish Water Eamon Gallen said: "Issuing refunds to 990,000 households presents a significant challenge but our priority is that the process is as efficient and straightforward as possible for our customers.
"Crucially however, we are reliant on them telling us if their details have changed since the last time they contacted us so we encouraging anyone who has moved or changed their personal details to contact us as soon as possible to provide up to date information.
"It is inevitable that some refunds could be delayed if information is not updated but our primary focus is that process is a simple as possible from a customer point of view."
The utility added that the best way for customers to ensure their details are correct it to call them on 1850 448 448 from Monday to Friday between 9am and 5.30pm.
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