Energia, part of power giant Viridian, is to invest €3bn (£2.7bn) in renewable energy in the Republic over the next five years, in a move that will create 200 direct jobs.
Viridian owns Energia in the Republic, and supply company Power NI in Northern Ireland.
Energia also supplies some businesses across Northern Ireland.
The investment programme will be implemented across a number of projects, including both onshore and offshore wind farms, solar power, hydrogen fuel generation, bio-energy facilities and the smart grid.
An estimated 3,000 to 5,000 construction jobs will be created over the five years.
Energia said the move would contribute to further sustainability in the Republic of Ireland's energy supply, and to the achievement of Ireland's climate change reduction targets.
The announcement comes two weeks after the Republic's government unveiled its Climate Action Plan.
The company anticipates it will add up to 1,500MW of renewable generation to the grid over the course of the programme.
Energia chief executive, Ian Thom, said the decision is "a clear signal of our intent to build on our continuous commitment to Irish communities, the economy and the sustainability of Ireland's energy supply, while contributing decisively to de-carbonisation and the protection of our domestic and global environment".
Energia, which has been operating in the Republic for 20 years, currently employs 800 people across the island of Ireland.
The Taoiseach, Leo Varadkar, said he "commended" the company for its investment.
Viridian was bought by I-Squared in 2016 for about €1bn (£0.89m) from Bahrain's Arcapita.
In the 12 months to March 31, 2019 Energia reported earnings before interest, taxation, depreciation and amortisation (EBITDA) of €167.1m (£149m).
The results were boosted by the performance of its renewables and customer solutions businesses.