Sinn Féin leader Gerry Adams has called for a "robust, energetic and vigorous campaign" against the Lisbon Treaty.
He said the treaty did not deliver reform, efficiency or democracy.
"It is a bad deal for Ireland. We deserve and can achieve better. The treaty will involve the most substantial transfer of powers from member states to the European Council and Commission to date," he added.
"The influence of smaller states will be reduced as the dominance of larger states is consolidated.
"It significantly accelerates the militarisation of the EU and advances an economic agenda based on a race to the bottom for wages and workers' rights."
Speaking to journalists in Dublin on Saturday, Mr Adams predicted that the referendum campaign would be difficult.
"However, I believe that we will be standing with the majority of people on this island," he said.
Mr Adams added that Sinn Féin would be the only Leinster House party advocating a No vote.
He said the Republic was a small state and should send a very clear signal. "We certainly want an EU which is different from that contained in the treaty," he added.
He was confident, he said, that a No vote could be secured.
"We did it before in the case of the Nice Treaty," he added. "Obviously, we cannot match the big parties in terms of financial resources, but we will mount a strong campaign."
Meanwhile, Labour spokesman on European affairs Joe Costello has called on the Government to publish the wording of the referendum and establish the referendum commission.
Speaking at a meeting of the Labour national executive committee in Waterford on Saturday, he said time was moving on if the Government was serious about holding the referendum in May.
"If the recommendation of the all-party Committee on the Constitution is heeded - that there should be a minimum of 60 days between the enactment of the legislation and the holding of a referendum - then the legislation would have to be passed by the early or middle part of March," he added.
