Three Ireland is to hike monthly phone bills for some customers by 25%.
The increase will apply to sim-only customers on bill-pay from June in a bid to even out pricing with O2 Ireland bills - which 3 Ireland recently took over.
Three Ireland said the increase would only affect about 1% of their customer base.
"We can confirm that there have been price increases for some of our services," the company said in a statement.
"The increases are essential to ensure continued investment in our network and services so that we can create a state of the art super-fast 4G network to suit our customers’ usage needs."
The company said it was committed to keeping a "competitive position in the market", and is seeking to "remain industry leaders on data packages."
A spokesperson later stated: "Less than 1% of Three customers are impacted by a €5 per month increase in one of its SIM-only price plans," the statement read.
"This increase reflects substantial network investment by Three following which the SIM-only price plan will remain one of the best value mobile packages available.
"Three will continue to assess its price plans aimed at enhancing its value proposition to customers and its value offering relative to other mobile operators."
Three has never made a profit in Ireland, and spent around €800m on buying O2.