The EU isn't insisting on the hard border. It was the UK who gave the suggestion that the RoI put in passport checks on entry - thereby annexing RoI to the UK. The RoI rejected that, so the UK wants a border to keep out those pesky EU migrants.
May has certainly tried to fearmonger the British people with a hard border. Of course, British negotiators told the EU they do not want one and would not build one. It was the EU that has insisted there be a hard border to stop the flow of goods into the customs union.
The Brits could always step up police checks of ID for people in Northern Ireland and impose a mandatory ID check to travel from Northern Ireland to the rest of the UK. That would be no problem for British citizens from Northern Ireland. The UK need not build a hard border they do not want. It would then be the EU’s problem to worry about the flow of goods into Ireland.