The Commission for Regulation of Utilities said today that all Iberdrola customers will automatically transfer to Electric Ireland and Bord Gáis Energy on Wednesday June 8 and will remain with those suppliers until September 1.
Customers can then switch supplier from that date.
The CRU also said that Iberdrola customers have the opportunity to switch suppliers up until Tuesday June 7 before the transfer to Electric Ireland and Bord Gáis Energy.
Earlier this week, Spanish energy supplier Iberdrola announced that it was exiting the Irish gas and electricity retail market.
It said the move follows an internal strategic review that found market conditions and pricing were a barrier to its planned growth and ambitions.
The firm added that the situation has been exacerbated in recent months by the unprecedented increase in global wholesale energy costs.
The CRU said that due to the volume of customers transferring - about 32,000 - the standstill period up to September will allow sufficient time for all electricity, gas and dual fuel Iberdrola customers to transfer successfully.
Iberdrola customers will transfer to the suppliers on their standard variable tariff, the CRU added.
It said that Iberdrola customers do not need to take any immediate action and their electricity and gas supply will not be interrupted.
Electric Ireland and Bord Gáis Energy will contact customers directly shortly after the transfer date to offer customers a tariff to remain with them or provide these customers with the option of switching to another supplier.