In the first instance id say for trade nothing changes and the whole of Ireland is treated as one, like it is now and how it will be if this deal is agreed.
On terrorism as per the article you put up, cross border security and the people of Ireland rejecting it will be needed and a zero tolerance from security services shown to the few that think they can commit terrorist acts, regardless of what side they proclaim to be on.
I never ever really thought I would see a United Ireland in my lifetime. Not that I saw it as being kept from us, just that when we voted down here to change our constitution and sign up to the GFA, any future settlement in my mind would have to be along the lines of dual citizenship. When talking of the Irish side having to compromise in a Brexit deal, I just feel that the full extent of the compromises from both communities in the North and the South are just not fully appreciated by those demanding it.
I didn't see a border poll arising, maybe never in the rest of my lifetime, or certainly not until the current generation of (rightly or wrongly) embittered politicians are gone.
A couple of generations of non sectarian living, I figured and then maybe approach it. But regardless of the outcome, I saw the unionist community of still having the self-determination guaranteed within the GFA. Some sort of dual citizenship would need to be accommodated. There would need to be a mechanism to opt back into the UK if later generations chose to. That to me keeps NI as a separate country which was palatable for moderate Unionism to sign up to.
I could be wrong but a Brexit referendum in the North on EU alignment seems in contradiction to this.
It's not an insurmountable obstacle. It could be addressed with the will to do it, I'm sure. But given the nature of how the GFA was meticulously put together can you not see that the time frame for realistic engagement on the subject is sadly lacking.
The unfortunate truth also is that there were many at the time who believed and stated that the UK government's word is worth nothing.
Rightly or wrongly whatever outcome is enforced upon NI will be seen by elements including those who were persuaded to compromise as a vindication of that viewpoint.
