Up to 150 jobs look set to be cut at Three Ireland.
The company has said it has made the difficult decision to enter a proposed collective redundancy process.
Most of Three's workers are based in Dublin but they also operate a centre in Limerick.
The company employs more than 1,300 people in Ireland.
In a statement, the company confirmed it will begin a collective redundancy process after a year of lower profits and rising costs.
Between 130 and 150 roles could be affected by the plans.
Three Ireland has already implemented a number of measures in a bid to reduce operating costs.
The statement added that the measures announced will position the business on a stronger platform for the future, ensuring it can maintain its market leading position and will enable growth.
Three has 45.5% market share in Ireland with over 4.3 million customers as of September 2023.