Users of Carphone's TalkTalk fixed-line service will be able to call other UK and Irish TalkTalk users free of charge, copying the strategy it has adopted in the UK to lure customers from BT.
'I think people are paying far too much for telephone calls in Ireland and we recognised this as an opportunity to introduce a cost-saving service,' said Stephen Mackarel, head of Carphone in Ireland.
In a statement, Carphone said: 'TalkTalk is so confident that Eircom customers will save money, it will give you Eu1,000 [£666] if you can prove otherwise.'
There are about 920,000 TalkTalk customers in the UK, as well as operations in France, Spain, Germany, Switzerland and Belgium.
Carphone directors David Ross and Charles Dunstone both sold stakes of 6 million shares in the business this month, but the founding pair retain large holdings in the business.