One of the reasons that the Brexit debate has been so fraught (or so it appears to us Yanks) is that the Tories are currently supported in Parliament by the Democratic Unionist Party. This party draws all of its support from Northern Ireland, and is firmly committed to having no distinction between Northern Ireland and the remainder of the UK. The Tories do have the word "Unionist" in their name as well, but as far as I can tell, maintaining the Union as it stands is not their raison d'être in the same way that it is for the DUP.
With that in mind: could there be a scenario under which some fraction of the Tories and other parties (Labour, the SNP, etc.) get together and vote through a motion that establishes customs borders in the Irish Sea? Is there sufficient support for this idea among the opposition parties? Or are all parties in Parliament pretty much united in drawing a hard line against such a separation?