It bought the stake from Australian investment firm Macquarie Capital, which has now divested its shares in the 56-turbine project.
The investment firm sold its remaining 12.5% stake in the 56-turbine site, which is nearing completion 30 km off the coast of Suffolk in south-East England.
Macquarie Capital had previously owned 25% of the project, but sold a 12.5% stake to Japanese bank Sumitomo Corporation in July 2016.
The remaining 75% ownership is still split evenly between three partners: Siemens Financial Services, Innogy, and a consortium of the Green Investment Group (GIG) – which is owned by the Macquarie Group – Macquarie’s European Infrastructure Fund 5 (MEIF5, for projects within the European Union, as well as in Norway, Switzerland and Iceland), and higher education fund, the Universities Superannuation Scheme (USS).
Financial close of ESB’s acquisition was completed on 29 March, but the price has not been disclosed.
ESB stated that Galloper, which consists of 56 Siemens Gamesa’s SWT-6.0-154 turbines, "is in the final stages of construction".
The utility’s chief executive Pat O’Doherty said the investment was a "significant first step" for ESB into the offshore wind sector.
He added: "Renewable electricity is a key enabler in the transition to a low-carbon economy and ESB expects offshore wind to play a pivotal role in this transition.
"Our investment in the Galloper wind farm underscores ESB’s commitment to further involvement in the development and construction of offshore wind farms off the coast of Ireland and Britain, as part of the transition to a brighter, low-carbon future."
ESB owns a portfolio of hydropower, thermal, biomass, and pumped storage stations, and also onshore wind farms. The majority of its projects are located in Ireland.
It has 508MW of installed wind capacity and a further 1,409MW in various stages of development, according to Windpower Intelligence, the research and data division of Windpower Monthly.
The utility aims to expand its wind power portfolio to 1.6GW by 2025, according to its website.