I think you have this the wrong way around. A Brexit deal concluded before 29th. March can be approved by weighted majority vote - In practice, so long as all the major players are on Board. An A50 extension on the other hand, requires unanimity. My take is that an A50 withdrawal - in practice an indefinite extension - would also require unanimity, although there is no express provision, precedent, or authoritative legal opinion on this point that I am aware of.

The same applies to any FTA or other association deal agreed after 29th. March, as the UK will then, effectively, be a third party. If any such deal involves an exercise of powers not already delegated by Treaty to the EU, it will also require a referendum in Ireland. It would be a delicious irony if the UK's future status vis a vis the EU became dependent on the vote of the plain people of Ireland in a referendum vote... I can see a lot of banter between relatives and friends in Ireland and the UK on the theme of "what will you give me in return for my voting in favour of your EU deal?"

Index of Frank's Diaries