No special deal possible to stop the return of Border controls
Checks are inevitable once the North joins the rest of Britain outside EU customs union
There have been some suggestions in Ireland, since the Brexit vote of June 23rd, to the effect that Ireland and the UK ought to be allowed to cut a special deal to avoid the need for a customs Border within Ireland. These interventions often seem to be ignorant of the basics of trade policy. Among the key points are the distinction between free trade areas and customs unions.
In a customs union, member states agree to a common external tariff, enforced along their common border with the rest of the world. Why would they do this, rather than retain the freedom to set their own tariffs vis-a-vis other countries? Because without a common external tariff, internal border controls would be necessary to avoid what is known as trade deflection.