It's certainly true to say that the Remain Campaign focused
primarily on convincing swing voters of a narrow proposition: that leaving the EU was too economically risky (this was the same approach that had been adopted successfully in the 2014 Scottish Referendum and 2015 UK General Election and was, in fairness, a rational response to the polling data at the time).
However, I don't believe the Irish issue was neglected quite as much as you suggest (perhaps I'm simply misremembering here). A quick news search returns
Theresa May asserting the inevitability of Irish border controls in the event of a Leave vote and Former Northern Ireland Secretary
Peter Mandelson warning of "renewed sectarianism", risking jobs and the entire NI peace process.
I am sure that you probably do recall the joint press conference between Tony Blair and John Major in Derry, which definitely achieved widespread media coverage for its symbolic significance. During it, Major warned, "the plain, uncomfortable truth is that the unity of the UK itself is on the ballot paper in two weeks’ time."
https://www.theguardian.com/politic...nd-john-major-brexit-would-close-irish-border
These issues should have been given much more prominence than they were, of course, but anyone who claims they genuinely had no idea that the Irish border would be a huge problem if we voted Leave, simply wasn't paying attention.