Households and small businesses are facing a 20% increase in gas prices this autumn.
The Energy Regulator has given Bord Gáis permission to increase prices from 1 September.
The Commission for Energy Regulation has also warned that a further increase will be needed in January due to the sharp rise in gas prices on international markets over the past year.
The CER described the increases as regrettable but inevitable.
The regulator said Ireland imports 90% of its gas from the UK, where the wholesale price of gas has almost doubled in the past 12 months.
It said the wholesale price accounted for 60% of customers' bills.
The regulator said the two price rises were an attempt to cushion the immediate consequences for customers.
It said the exact level of the second price rise, to be announced in November, would depend on developments in the gas market.
Gas prices in Northern Ireland rose by 28% in May.
The Society of St Vincent de Paul has said it is very concerned at the price increase, which they say will impact hardest on the most vulnerable.
The society expressed the hope that Bord Gáis will not include the Standing Charge in their price increase.
The society also called on the Minister for Social and Family Affairs, Mary Hanafin, to tackle the issue of fuel poverty among those depending on social welfare payments.
Meanwhile, Age Action has called on the Government and energy providers to take steps to ensure that vulnerable older people are protected from rising fuel.
Eamon Timmins of Age Action said: 'This increase along with the recent increases in the cost of oil and electricity leaves many older people praying for a mild winter. They know if the winter is hash they will suffer.'
