There are concerns the Government will have to commit to an annual 3 billion euro defence bill if they join PESCO.
The deal would see EU states pool their defence forces and raises issues surrounding Ireland's neutral stance.
The Peace and Neutrality Alliance says the extra military costs involved will hamper the housing recovery.
Executive Member, Carol Fox, thinks its scandalous to spend more money on militarism and says the Government hasn't released details on how they intend to pay for the deal.
She said: "It could require quite a big increase in our defence spending if we are supposed to bring our capabilities up to certain levels then also we will be committed to proming the arms industry through the European Defence Agency and through other group agencies aswell.