The Green Party and the human rights organisation, AFrI, called for a referendum on Irish membership of the force.
The Green TD, Mr John Gormley, said people would now see they had been "totally misled" by proponents of the Amsterdam Treaty, including the Government, which had "vehemently denied that the signing of this treaty would lead to a European army".
He added: "Our reputation as a neutral country has been severely undermined. This is a terrible blow, not just to Ireland's neutrality but to our well-earned reputation as UN peacekeepers. Just at a time when we win a seat on the UN Security Council, we are also undermining the UN by shifting our troops to a Rapid Reaction Force serving the EU."
AFrI said the Government was being "deceptive, dishonest and undemocratic in its decision to pledge Irish soldiers and equipment to the European Rapid Reaction Force". It was "a major shift" in foreign policy, but it also continued a trend of moving closer to NATO and away from the UN.
AFrI spokesman Mr Joe Murray said the decision was at odds with Ireland's emergence from three decades of conflict.