The mobile phone company O2 Ireland has been bought by Three in a deal worth up to €850m.
Three are paying €780m up front and will pay a further €70m depending on O2's financial performance.
O2 is owned by the Spanish telecoms company Telefonica and has been selling assets to pay down its debts.
The combined companies will have nearly two million customers and a 37.5% share of the mobile market if the takeover is approved by competition watchdogs.
CEO of Three Robert Finnegan said that this merger is good news for customers.
"The opportunity that we are given by combining these businesses is actually also from a network point of view," he said.
"We will be able to invest significant amounts of money in building up what is going to be a super network within Ireland and bring the latest technology to people here."