Airtricity energy customers are facing huge hikes in their bills next month with electricity going up 12.3% and gas a massive 21.2%.
The company blamed increases in wholesale energy prices and warned that it could no longer absorb the costs without passing them on to the household market.
Kevin Greenhorn, Airtricity chief executive, said the company regrets having to increase energy prices in "these difficult times".
"Airtricity has sought to lead the way in price reductions and bring competition to the Irish electricity and gas markets," he said.
"It gives us no pleasure to announce our first price increase since joining the mass domestic market in 2008.
"Unfortunately there has been a significant increase in wholesale energy prices, and while we have done our best to absorb these costs we can no longer do so.
"In recognition of increased pressure on living costs we will continue to support our customers in innovative ways to budget for this increased cost."
Airtricity has 482,000 household customers in the Irish market.
The company said the price hikes for electricity will take effect from September 1. The gas prices rise from October 1.
Airtricity said electricity customers can expect to pay on average 2.53 euro extra a week or 131 euro a year and customers using both gas and electricity supplies will pay an additional 5.45 euro a week or 283.40 euro a year.