Regardless of the DUP's current influence, it really shouldn't have been much effort to realise that the NI issue could be a sticking point. My thoughts at the time were a bit less complicated than the current situation (I assumed that the EU would be wanting changes involving a "harder" border allow an actual EU border for people, goods and services and the UK government would be the one sticking up for the GF agreement - even I wasn't informed enough to know much more than that), but it really isn't that difficult to put two and two together. Even if you aren't aware of the details of the NI issue, the concept of the NI border being a soft one and that maybe being an issue if we leave the customers union or stop freedom of movement. Of course, back in the real world what happened was leavers made a lot of noises about either staying in the CU, or having a Norway type deal, or even staying in the EEA, all of which would have made the NI issue a much much smaller one, so I didn't really pay TOO much attention to it - but I damn well knew that the issue could blow up - if you weren't aware of that then you had no business voting in the referendum at all IMO (which is why I opposed the referendum in the first place, too many people not knowing or understanding the consequences).