3 Ireland is to hike monthly phone bills by 25pc for thousands of customers as it tries to even out pricing differences with O2 Ireland bills.
The price rises will apply to bill-pay customers on 'sim-only' deals from June.
The monthly hike - from €20.33 to €25.41 - will raise concerns that the operator's recent €850m takeover of O2 Ireland could now lead to higher mobile bills in Ireland, a fear voiced by Ireland's communications regulator and by some in the European Commission.
The operator may also scale back some mobile data allowances for prepaid users next month, as it seeks to further synchronise pricing plans between its two customer segments.
Up to now, O2 Ireland bill-pay tariffs have been more expensive than those of 3 Ireland, as 3 tried to aggressively lure customers to its network. But the operator is on a fresh drive to try to break even in the Irish market, after losing more than €1bn since it set up here.
Earlier this year, 3 Ireland absorbed 1.5 million O2 Ireland customers as it completed the acquisition of the larger network.
A spokeswoman for the operator said the price rises were required to meet its current running costs.
"It's essential that we continue to invest to create a state-of-the-art 4G network, with enhanced data and network quality for our customers to enjoy," she said.
"We are constantly reviewing our price plans to ensure that they are competitive and offer value to our customers. Rest assured, that all of our plans continue to offer excellent value."
3 Ireland is now the second biggest mobile provider in the country. It has a 32pc market share, with market leader Vodafone on 39pc. Two new mobile operators are due to begin services later this year, with The Carphone Warehouse launching this summer and UPC launching in the autumn.