Tánaiste and Labour Party leader Eamon Gilmore has claimed that it cannot be “business as usual” if Ireland rejects the fiscal treaty.
Speaking at the formal launch of the Labour referendum campaign, Mr Gilmore said a No vote would undermine investor confidence and cut Ireland off from emergency funding from Europe should the country need it.
However, the Workers' Party has said the "tide is turning" against the fiscal treaty in the referendum campaign.
Ireland's European Commissioner Máire Geoghegan-Quinn earlier warned that it was "absolutely the case" that Ireland would not have access to ESM funding if the fiscal treaty was rejected.
She said it was "vital" that people vote yes.
Speaking in Brussels, Mrs Geoghegan-Quinn claimed: "There's isn't a Plan B. There isn't a way of renegotiating the treaty to get a better deal, because all this treaty needs is 12 member states to ratify it.
"I want to make sure that my country is part of that 12, is involved in ratifying the treaty and will have access to money if it’s necessary to have that access at any time in the future," she said.
Describing the vote as the most important referendum on Europe that Ireland had ever faced, she said a Yes vote was about "jobs, investment, certainty”.
Mrs Geoghegan-Quinn said: "Above all, this treaty will be about jobs and money. It will be about ensuring that Ireland can access funding in the future, regardless who is in government.
"I'm also very concerned that people who create jobs in Ireland in every corner of the country will - by the people voting yes - be ensuring for the future, certainty for the investment that is already there and making sure there's certainty for investment for the future."
Workers' Party launches No campaign
The Workers' Party launched its campaign for a No vote this morning, saying the Constitution was not the place to enshrine economic policy.
Party President Mick Finnegan said the French presidential election result was a resounding rejection of austerity.
Mr Finnegan said the treaty would lead to more unemployment.
European spokesperson Padraig Mannion said the No side was at the same point as it was in the first referenda on the Nice and Lisbon treaties with three weeks to polling day.
Mr Mannion also criticised Mr Gilmore for travelling to Paris to celebrate the victory of Socialist Francois Hollande, while pursuing different policies at home.
He said that in France the Tánaiste welcomed the election of a candidate opposed to austerity and committed to changing the treaty, while at home he urged people to support the same treaty.
Elsewhere, European Union leaders will meet on 23 May for talks expected to focus on how to boost growth.
President of the European Council Herman Van Rompuy announced the date for an "informal dinner of heads of state or government" on Twitter.