You can make a binding promise that there will be regulatory convergence in Ireland.
And you can promise no border in the Irish Sea to the DUP.
As you can also promise to leave the customs union to the people of the UK.
But in the process you guarantee three things.
First you are guaranteed to look like a fool because the three are incompatible.
Second, you are guaranteed to break your promise to someone as a result, and you can be sure they will be annoyed.
Third, you can guarantee that the person to whom you made the binding promise will pull rank and say you must honour the commitment. The EU did that on the issue of the Irish border yesterday. They were right to do so.
May has made many foolish and expedient promises in the short time she has been in office. Ireland is now calling her out.
What happens now? I wish I knew. But sometime soon this is going to be very ugly. There is no political way around that.