Borders have two sides
You can do what you want at your own border, and the person on the other side can do what they want to. So you can have no checks at Dover - the French can check every entering lorry in a process that takes 24 hours per truck. Good luck with that. Think of a fence between two gardens. You may not have built it, you might not want it, but if your neighbour put it up on their land you are still going to get a sore nose if you try and walk through it.
Though actually in your no-deal-WTO nirvana, you can't discriminate between countries. So if you want WTO trade terms, free movement from Ireland means free movement from Botswana, in trade terms.