The Bord Gáis Energy index for September registered a 1% decrease.
Oil prices fell slightly last month after world powers made efforts to address the recent record highs of oil prices.
The Bord Gáis Energy index stood at 150 in September, up 11% on the the same time last year after a year of rising oil prices.
Despite last month's fall in the price of Brent crude oil, the price has risen by 26% since June and remains at record highs.
The natural gas element of the index was up 11% in September, despite the fact that UK gas demand was below seasonal norms due to mild weather and reduced demand from gas-fired generation. Prices rose due to reduced gas supplies from Norway and market fears over the availability of future supplies of LNG from the Middle East.
Today's index shows that its coal element was down 7% as global coal supply chains resumed as normal in September after strikes in Columbia and Hurricane Issac in the US. A weakening US dollar versus the euro, falling oil prices and softer Germany wholesale electricity prices also lead to the fall.
The electricity element of the index rose by 5%, due to higher gas prices. The majority of the electricity produced in Ireland is generated by burning imported gas from the UK. Despite historically low gas demand in the UK, supply struggled to keep up in September due to maintenance outages at producers in Norway and Qatar.
The oil element was down 4% last month due to speculation that a drawdown of emergency oil stockpiles was imminent, which helped pull downward pressure on prices. Other factors hitting prices included a fall in Chinese oil demand, improved North Sea production and the resumption of oil exports from South Sudan and the Gulf of Mexico.
John Heffernan, a power trader at Bord Gáis Energy, said that a rare statement from the G7 finance ministers calling on oil producers to increase their output in order to ease the ongoing pressure on oil prices appears to have been somewhat successful.
He said reassurance from OPEC appears to have had a calming effect on the markets and led to the slight reduction in oil prices.
But oil prices remain under pressure due to various geopolitical developments, especially in relation to the Middle East and concerns about Iran's nuclear programme.
He said that as future wholesale gas prices in the UK have a strong relationship with the price of oil, geopolitical developments over the coming months are as important to the UK and European gas markets as global oil markets.
''As we approach winter, weather and gas supplies to the UK will also heavily influence gas prices,'' he added.
