GardenGeek Its not like people and goods can just magically appear there - they have to leave somewhere with a border or arrive somewhere with a border surely
What am I missing here?
There is a huge difference between a barrier (some sea, a river, a range of mountains) and a border. If there is a border there has to be customs&excise inspecting the goods passing from one area to another and examining the documentation of the people travelling across to ensure that they have the right to be in the place they are travelling to. That takes time and costs money but the only way to avoid it is to have an agreement between the two areas that there will be free trade and free movement of people between the two areas. Having such an agreement in place means you don't need to spend time and money on maintaining the border and the economies on both sides thrive because of this.
Currently it is no more hassle to get a ferry over to France than it is to get a ferry over to the Isle of Wight, or between Belfast and Liverpool. There may be a security check keeping an eye out for explosives and weapons but that is a tiny delay in comparison. Obviously you still have to cross the barrier but there is no queuing after the crossing for border guards to check that you have the authority to transport whatever/whoever you are into the destination area, no lengthy forms or tax/duty to pay on goods - these taxes are obligatory under World Trade Organisation rules unless there is a free trade agreement in place. There is also no need for patrols on land or sea to prevent any crossing that isn't authorised and controlled.
Ireland needs this beneficial arrangement of a free trade agreement with the mainland EU. The UK has absolutely no right to take that away from them.
Northern Ireland needs this both between them and the Republic of Ireland and between them and the rest of the UK. This is a fundamental part of why we have had more than 2 decades of peace in Northern Ireland after decades of there seeming to be no solution. The Good Friday Agreement could not have worked without this as it allowed both sides of those awful years of conflict to achieve a measure of union with the land they identified with, be it the Republic or Ireland or the UK. It meant that conflict could end with neither side being "winner" or "loser" but both sides compromising for peace.
Now whether a border is established between NI and the rest of the UK in the sea, or a land border between the north and south you would be saying to one side or the other "actually guys, you lose, you can no longer have the union you fought for". Obviously that cannot happen.