Looks like about 20 years worth of expensive and difficult negotiating to me.
The Irish question means that whatever anyone suggests re borders, customs and immigration will come up against big problems
Free movement between Irish Republic and UK dates back to a time before the two countries joined the EU. This includes the right to vote in all elections, which is different to the rest of the EU. The open border between N and S is part of the Good Friday agreement.
And as to customs - a lorry full of N Ireland beef, travelling to France, via Dublin port, would have to cross a border into the South, then another border into the UK at Holyhead, then another border at the French coast. Currently they can drive through with no customs tariffs or paperwork. The ports are not designed to cope with long queues.