scooby wrote:Is a hard border in Ireland for checks on goods or people or is it both?...
The problem is mainly goods. It's easiest to think in terms of concrete examples.
Most countries want to impose regulations that goods must meet before they can be sold. It's a choice, there is no god given commandment that they must do this. For example 230 V mains electricity is dangerous. It can kill you. That means that any product that uses 230 V mains electricity has the potential to kill you if it is not designed correctly and built to a correct standard. Virtually all countries have regulations that such products must meet before they can be sold to stop consumers being killed. It is possible to make an argument that such regulations shouldn't exist. Meeting those regulations is a cost to the manufacturer so the product costs more than it would if he didn't need to meet them. If the regulations were scrapped all consumers would benefit from lower prices and only a few would probably pay the cost by being killed. Which is best? Every consumer paying a bit more and nobody dying or every consumer paying a bit less and a few people dying? Everybody makes their own judgement.
Regulations in different countries are not necessarily the same even for the same product. As another example, the EU has decided that injecting cattle with huge quantities of anti-biotics and growth hormones is not a good thing to do. They fear it might be a health hazard to consumers and so have regulations in place to control it. All cattle raised anywhere in the EU must conform to those regulations so cattle and meat products from them can move and be sold anywhere in the EU. The USA has different rules. Their government has decided injecting cattle with massive quantities of anti-biotics and growth hormones is fine and they don't have regulations restricting it, or at least not to the extent that the EU does. This allows US cattle farmers to be more productive than those in the EU and so they would have a competitive advantage if they could sell US standard meat products in the EU. But to sell their products in the EU they have to meet the EU standards so they can't use the same levels of anti-biotics and growth hormones so they don't have a competitive advantage. When the UK has left the EU and does a trade deal with the USA it will probably have to concede that beef produced to USA standards can be sold freely in the UK. If there is no customs checks between Northern Ireland and the Republic the USA could then send Beef to Belfast ship it into the Republic and from there to the rest of the EU.
Either the UK maintains the EU regulations, as a whole or just in Northern Ireland and no checks are needed or it doesn't follow EU regulations and checks are needed.