More than a million electricity customers are facing higher bills after Electric Ireland said it was hiking prices next month.
The supplier's move to raise prices by 3.4pc will add around €35 to the average bill over a year.
Its rivals are also expected to increase their prices, an expert said.
Electric Ireland's price rise from October 1 comes after the firm had reduced prices by 2.5pc in April.
Its gas price is to remain the same for the winter months.
The state-owned company said the increase in electricity prices is due to recent costs affecting the broader electricity system and market.
The electricity price rise is set to hit at a time when households use more energy.
October 1 is also the date when households and small firms will see a rise of more than double the levy charged on their electric bills to support the development of renewable energy supplies.
The public service obligation (PSO) levy is set to rise by 130pc, adding €44 a year to domestic bills.
"In April, we reduced energy bills for gas and electricity customers by circa €100 annually because fuel costs were falling at that time," executive director of Electric Ireland Marguerite Sayers said.
"Unfortunately, due to other electricity system and market costs outside of our control, we now have to increase our electricity prices from October 1."
She said the increase will mean an increase of €2.88 a month on the average residential electricity bill.
David Kerr, of price comparison site Bonkers.ie, said: "This is likely to be the first of several increases by suppliers.
"The price change comes on the same day that the PSO levy jumps a massive 130pc to €96.24 per year and will be unwelcome by all electricity customers, as no customer can avoid the PSO levy increase."
The latest price rise comes as the numbers switching to new energy suppliers fell again last year, even though there are more than a dozen providers.
The annual report from the Commission for the Regulation of Utilities shows prices here are higher than the average across the European Union.
The average price in Ireland for electricity consumers was 7pc above the EU average for January to June 2019.
There have been calls to make the switching process easier to help households avoid overpaying for their electricity.