In the interests of clarity, there are really (at least) four distinct issues with the border crossing. The first is simply whether people have a right to phys

In the interests of clarity, there are really (at least) four distinct issues with the border crossing. The first is simply whether people have a right to physically cross the border. This is, broadly speaking, the Common Travel Area that was established on the island of Ireland long before the EU or any of its predecessors existed. The next is Freedom of Movement, which is one of the "Four Freedoms" that underpin the EU Single Market. This is also about people, but rather than physical movement, it relates to what they can legally do. Notably, it grants someone from one side of the border rights to live and work on the other. The third is customs. This is about moving goods across the border, whether tariffs are payable to do so, and the mechanisms for monitoring and enforcing any such tariffs. At present, with the UK and Ireland both in the EU and its Customs Union, there are no tariffs and so no checks or infrastructure are needed. The fourth is regulatory compliance. This is also about moving goods across the border, but it's about the non-tariff barriers, such as whether products for sale meet environmental or safety standards, or whether live animals or food products meet health requirements. These matters are typically regulated at the level of the EU's Single Market, so again, with the UK and Ireland both in the SM there are no discrepancies in standards and any accreditations for testing on one side are recognised by the other, meaning no need for additional verification at the point of crossing the border. Of course there are also other issues with cross-border trade; notably, the above doesn't cover anything to do with provision of services. But in terms of physical crossings, those are the main points, and they are quite distinct.