The HSE spent over €1.5m on agency staff at UHG and Merlin Park in the first quarter of this year.
The figure was revealed at a recent meeting of the HSE Health Forum West following a question from Councillor Shane Forde.
He was told the HSE is also struggling with an over-run in the local health budget to the tune of €24m, or 4 percent.
Management said controls are now being introduced to reduce dependence on agency staff, and a wider savings plan is also being implemented.
Fine Gael Councillor Forde says the reliance on agency staff is not sustainable and must be tackled.