The EU have committed to Ireland, and the Irish government themselves have committed that there wouldn’t be a hard border. The U.K. government has also committed not to build one. It’s not just the U.K. that has committed not to build one - it’s all parties involved in this that have committed not to build one.

In addition, there are other borders between EU and non-EU countries that don’t require immigration controls. It’s hard to see why one is necessary - other than if the EU was deliberately setting out to make leaving look as difficult as possible.