Virgin Media Ireland is to offer its services to a further 450,000 households, businesses and other organisations around the country following a deal with SIRO.
The ESB-Vodafone joint venture is rolling out a fibre broadband network across Ireland.
Under the agreement, Virgin Media Ireland will be able to access it and offer its products and services over it.
The move will greatly expand the reach of Virgin Media Ireland, whose own network is capable of offering speeds of up to 1Gbps to a million premises.
"Today's announcement increases our network reach to 70% of all the premises in Ireland," said chief executive of Virgin Media Ireland, Tony Hanway.
"Our new partnership with SIRO means we will be offering our market-leading ultrafast broadband and TV services to more Irish consumers and businesses than ever before."
SIRO plans to grow its network to a total of 154 towns and cities, reaching 770,000 premises and 2.1 million people.
It is currently capable of offering speeds of up to 2Gbps to residential customers and following a planned upgrade this will rise to 10Gbps.
Virgin Media Ireland will become one of 20 retailers who resell access to the SIRO network and offer services on it.