An alliance of 15 groups have jointly launched their campaign for a No vote to the Lisbon Treaty.
The new alliance includes Sinn Féin, the People Before Profit Alliance and the Socialist Party.
Socialist Party MEP Joe Higgins described the Treaty as 'profoundly undemocratic', at the launch of the campaign.
He also rejected claims the guarantees obtained by the Government would protect workers' rights.
However, Fianna Fáil MEP Pat the Cope Gallagher has described the comments by Mr Higgins as 'disingenuous and wrong'.
He said there was a changed basis for the holding of the referendum, despite what Mr Higgins said.
He added that the Government had legal guarantees from EU leaders, that Lisbon would not affect Irish policies on military neutrality, taxes and abortion.
53% of the electorate rejected the treaty in the first referendum in 2008.
A second referendum will be held on 2 October.