There will be a border. There is at present. It can remain the same unless the EU are determined to cause trouble. In which case, having decided a closed border is to be insisted on they will instruct Eire to stop the open border arrangement despite the UK upholding their side of the agreement to leave it open. Those responsible will inevitably be Eire and their EU masters.
As for Barnier, he's just causing trouble whilst trying to look as if butter wouldn't melt in his mouth. Basic EU control tactics. In this case trying to force NI, and thus the UK, as one part can't be in whilst another's out, to stay in the EU in all but name. As a bonus for Eire, the daft negotiation suggestion to avoid the issue, might encourage further demands for a united Ireland. Not that any Westminster politician should give it even a moment's consideration. As I said, trouble making.