The utility company will be refunding almost a million customers over the coming months
Irish Water customers are being urged to update their details if they've changed address since paying the last water bill.
The utility company is set to refund around 990,000 customers over the coming months.
While the Government must first pass legislation and provide necessary funds to Irish Water, the refund process will be automatic for any customers who paid their bill.
However, the water company says it is 'essential' that all details are up-to-date to avoid any delays.
Irish Water's Eamon Gallen explained: "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."
He added: "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."
He stressed that the company had already done a 'significant' amount of work to prepare for the refund process.
More details about refunds are available on the Irish Water website.
The Government has indicated that the vast majority of customers will be refunded before the end of the year, although some "exceptional cases" will be processed early next year.
A household of two adults who paid all five water bills will be due a refund of around €325, while a single adult household who paid all bills will be due €200.
The water conservation grant is unaffected by the refunds, and the Government will not be looking to recoup the €100 grant.