Airtricity, which is owned by SSE, has agreed a deal to buy Phoenix Supply Limited, Northern Ireland's biggest gas supplier for £19.1m sterling.
Following the deal, Airtricity - SSE's retail supply brand - will become the North's second largest energy supplier and a dual fuel provider of gas and electricity to over 230,000 customers.
The acquisition is subject to approval by the Irish Competition Authority and SSE expects to complete the deal during the summer.
Airtricity entered Northern Ireland's competitive domestic electricity market in 2010.
Over 100,000 Northern Ireland domestic and commercial electricity customers have switched to the firm since then.
Phoenix's gas supply arm is made up of three companies - Phoenix Supply which supplies gas to around 130,00 customers in the greater Belfast area, Phoenix Energy which supplies gas to large industrial and commercial customers in Ireland and Phoenix Energy Supply which was set up to supply gas in the ten towns licence area outside the greater Belfast area.