Prepaid mobile customers of O2 and Meteor face an 80% increase in some call charges.
The changes will affect around 1.5m prepaid users across the two companies.
The Consumers Association of Ireland has said the charges are totally unjustified as Ireland has amongst the highest mobile costs in the EU.
Both companies are increasing the call connection charges from 5c to 9c.
Those on O2 will pay more from today, while the Meteor price hikes will come into effect on 12 August.
