ESB, the main electricity company in the Republic, has bought Northern Ireland Electricity (NIE) for £1.2bn.
The Irish state-owned firm said on Tuesday it had finalised the details of the takeover.
A joint statement from the ESB and NIE's parent company Viridian said: "NIE will continue to operate as a stand-alone business under the NIE brand and identity."
ESB announced plans to buy NIE in July.
Its chief executive Padraig McManus said: "We are delighted with the acquisition of NIE, an efficient and successful company.
"ESB is committed to making the capital investment required to match future growth in the economy and to meet the Northern Ireland government's targets in relation to the development of renewable energy."
Patrick Haren, Viridian Group chairman, said the sell-off would see the firm, which employs 1300 people, provide more renewable energy to customers.
He added: "Under Viridian Group's ownership, NIE has strengthened its network capacity and improved its service delivery to customers significantly.
"Under its new owners, it will meet the challenge of grid development and delivering increasing volumes of renewable energy to customers.
"We wish our colleagues at NIE well for the future. Viridian Group, under Arcapita's continuing ownership, will now be focused on our very significant generation and supply businesses."
The sale faced a number of setbacks earlier this year, including financial complications with Viridian and opposition from unionist politicians.
Leader of the DUP Peter Robinson and the former head of the UUP Reg Empey wrote to the Irish government objecting to ESB's plans, claiming the deal could have major political implications for Northern Ireland.
But the leader of the SDLP, Margaret Ritchie, welcomed the move on Tuesday.
She said: "This is practical, mutually beneficial north-south co-operation which must be a model for the development of cost-effective infrastructure."