Fianna Fáil has written to the Minister for Finance, warning him about possible changes to plans for a "rainy-day fund".
The move comes after suggestions that the new Finance Minister Paschal Donohoe could scale back his predecessor's plans for a Government savings fund.
The deal between Fine Gael and Fianna Fáil requires the Government to set up the fund, but it has been reported that much of the cash could now be set aside for big-ticket spending like the Metro North project.
Fianna Fáil's Michael McGrath said that the rainy day fund will be undermined if it does not have enough cash.
"We'll have to see how the final number falls, but from a Fianna Fáil point of view, the amount of money going into a rainy-day fund from 2019 onwards has to be more than tokenistic, it has to be of a certain quantum which, over a short number of years, adds up to an amount that is tangible and meaningful."