Energy prices including oil, gas, coal, and electricity fell by 6% in May, according to a recent report.
The latest energy index from Bord Gáis cites a 6% fall in oil prices and in electricity costs as sources for the overall decline.
Despite its fall month-to-month, the index stood at 88, a 6% increase year-to-year.
"OPEC’s November 2016 agreement has so far failed to halt the slide in oil prices, but the extension to this agreement announced at its end of May meeting could potentially see a reversal in prices later in the year, although this will depend to a large degree on whether the US continues to increase its production", Bord Gáis energy trader Darragh Crowley said.
Oil prices finished at $50.31 a barrel, $1.42 lower than April, while gas prices remained steady in May.
"Wholesale natural gas prices have soared in the past few weeks over uncertainty around gas supplies from Qatar to Europe", Mr Crowley said.
Wholesale gas prices remain 14% higher than the comparable period last year.
Coal settled at $76.55 a tonne and wholesale electricity prices fell by 6%, according to Bord Gáis.
The report says Irish wholesale electricity prices typically tend to track the cost of imported gas as it is the most significant cost in the production of electricity.