Fianna Fáil has written to the Minister for Finance urging him to ensure there is sufficient funding available to support sectors that would be hardest hit by a no-deal Brexit.
The party's finance spokesperson, Michael McGrath, also wants Paschal Donohoe to set out how the Government would approach a further Budget extension.
This correspondence comes just ahead of the publication of the Government's summer economic statement tomorrow.
The Government is preparing two budgets, which will be based on whether the UK will leave the EU at the end of October with or without an agreement.
The Government's summer economic statement is due to be published tomorrow and will include plans for two budgets - based on whether or not the UK will leave the EU at the end of October or not pic.twitter.com/0abZxwhyv7— RTÉ News (@rtenews) June 24, 2019
It will decide close to Budget day on 8 October which option to go with.
"If the Budget is not based on a 'no-deal' Brexit and that is what subsequently happens, it will be important to have sufficient headroom available to be able to provide targeted supports to the sectors of the economy worst hit by Brexit," Mr McGrath wrote.
He also said that given the immense uncertainty posed by Brexit and the need for political stability, Fianna Fáil would enter the Budget talks in good faith with a view to "facilitating the passage of a Budget in October".
Speaking on RTÉ's Six One News, Mr McGrath said the minister will have to adopt a cautious approach in the weeks coming up to the Budget if there is still uncertainty about the outcome of Brexit negotiations.
Admitting that the minister will be in a difficult position, the Fianna Fáil TD said if there is any uncertainty then Mr Donohoe will have to stick to the advice from the Fiscal Advisory Council and, in broad terms, to what was agreed in the Stability Programme Update in April.
He said the minister will need to hold something in reserve in case there is a need to introduce supplementary measures in the event of a hard Brexit.
Mr McGrath said that at this point it is very difficult to estimate the impact of such an outcome but a targeted intervention would be required to support certain sectors of the economy that are most exposed to Brexit, such as indiginous manufacturing and the agrifood industry.