The Irish Campaign for Nuclear Disarmament (ICND) has argued for a No vote to the treaty because it would conflict with Ireland's traditional status of neutrality.
The proposed military activities of the EU were linked to NATO, it said in a submission to the Referendum Commission, and would involve Ireland in an alliance whose strategy involved the threat of the use of nuclear weapons.
ICND said Ireland's contribution to peacekeeping could best be undertaken through the UN, as in the past. It said the definition of neutrality as "the lack of a mutual defence commitment" had only been introduced by the Department of Foreign Affairs in 1996 and was not part of the traditional definition.