Barnier is just 'window-dressing' for the other 24 states, a sop to them, to pretend they are somehow being consulted, or he may become a convenient 'patsy'(for both sides) if things do go pear shaped!
Germany, France and Britain will seek to sort this between them, its likely to require Britain to pay its leaving bill (if they can ever agree on what should be on it) and to give a contribution towards other matters, that will ensure solvency (i.e. nobody else paying anymore in or drawing any less out) for the EU for perhaps up to a decade, may depend on how long it will take for Germany and France to drive though to the new European Super State.
Estimated cost £80B, to paid over a decade e.g. £8B annually, in theory saving Britain £1-2B per year, at least that's how Theresa will probably try to present it to the nation (and to mollify Boris), its the 'best she can do', she will say. May will then, return and 'fall on her sword' more or less immediately like Cameron did, or when the Tory party 'grandees' order her to do so!
In return for all this guaranteed 'dosh' Britain will be granted access to the EU markets more or less as now, similarly the EU to the UK Market, with everything else more or less 'as is', there may have to be some special court of justice, set up that's made up of British legal eagles and the ECJ, to agree to oversee the rights of those citizens of both EU living here and Brits living in the EU countries. The Irish will sort their own border issues, as they have always (in reality) done and Britain and the EU will 'sandbag' whatever agreement is reached.