First, the N.Ireland border. You say that the Brexiteers want a hard border. Nobody, as far as I know, actively wants a hard border - even the DUP admit the advantages of the open border under the Good Friday Agreement. But they want the only alternative - a customs barrier in the Irish Sea between N Ireland and the UK mainland - even less. It would, in their view, fatally undermine the territorial integrity of the UK. So for the Brexiteers a hard border is the lesser of two evils.
Second, and perhaps more important, immigration. You're quite right to say that for a large number of 'Leave' voters immigration was the dominant issue. But it was not, as you say, immigration from the EU. This was only significant in a few parts of the country such as East Anglia, where a large influx of unskilled East Europeans to the food packing industry and seasonal agricultural labour has been highly visible and has created social tensions in a low population density environment. Elsewhere, EU immigrants typified by the 'Polish plumber' and other highly-skilled tradesmen has been generally welcomed (and incidentally has brought new life to many an ailing Catholic congregation, but that's another story...)
No, the issue for Leave voters in the country at large was immigration in general, particularly immigration from the third world. Now you and I both know this is nonsense, since it has little or nothing to do with the EU. But such was the lamentable lack of knowledge about the EU among the general public, and such was the effectiveness of the Leave campaign in stoking these fears without needing to make explicitly false claims, that very many people believed - still do believe - that the EU was somehow behind it, and that leaving the EU would somehow stem the flow. The realisation that third world immigration has hardly changed at all is hardly going to lighten the dark public mood.
NI. You have put it quite correctly, thank you
Immigration - correct. And third world immigration somehow seems to have caused people to vote against remaining in the EU