The Bord Gáis Energy index fell 2% in December due to a weaker US dollar and marginally lower Brent crude oil prices.
December also saw lower Irish electricity prices.
The index is designed to measure prices in the wholesale energy market. It tracks oil, gas, coal and electricity - all relevant prices to the Irish economy, Irish companies and consumers.
It stood at 149 in December, up 4% on the same time last year.
The oil element of the year fell by 2% after the release of a report in the US which emphasised the growing strength of the US as a major energy producer. Bord Gáis noted that the US reached oil production levels not seen in 15 years, mainly due to the increased use of hydraulic fracturing.
In December, the natural gas element of the index rose by 1% after UK wholesale prices increased as a result of the cold weather in the first half of the month.
In contrast, the coal element of the index fell by 1% due to an improved weather outlook for Europe in the short term, speculation that Europe will receive six cargoes of coal from South Africa in January and weaker global coal prices.
The electricity element of the index decreased by 5% despite rising wholesale gas prices in the UK and rising electricity demand in early December.
Bord Gáis energy analyst John Heffernan said that oil prices in 2013 will continue to be dominated by developments in both demand and supply.
''With oil demand being largely a function of GDP growth, developments in the global macro-economy will play an important role in determining the price of a barrel of oil. With the US, China and Europe accounting for over 55% of global GDP, and nearly half of the world's oil demand, what happens in these three economies in 2013 will be of vital importance,'' he added.