What are you smoking? If the UK leaves without agreement with the EU.. a plausible scenario, then there would be a trade war. There would be a hard EU border along the Newry road , policed by irish troops, augmented by the EU boarder police., And paid for by an additional supplement from Brussels to Dublin.. That's what no agreement means. The Irish government will have limited say.
Meanwhile the UK will have similar numbers of police, customs etc on their side , paid by the UK government. They , the UK government,will then be charged with designing and implementing a customs clearing regime... It's all horrible to contemplate.