Except for there own self interest and WTO rules but appart from that no one can force them .the British government have repeatedly said they will not enforce the hard border.
They don't want too and no one can force them too.
Whereas Eire will be duty bound to enforce the EU single market or otherwise all UK product could quite legally travel via Stranraer to Europe.
Ireland is not duty bound to enforce the EU single market . Ireland has a choice Either stay in the Single market or leave it. This choice is imposed on us by the British. But when bombs start going off again due to British decisions I'm sure people who get caught up in wont really care about who is maning the border.
In a no Deal scenario the most likely outcome is both sides trying to man a border that cannot be policed soliders back in NI and bombs going up all because the British didn't sign up to the WA all because the British decided to rip up a peace agreement all because the British did'nt negociate in good faith.
But you keep blaming the Irish for having a clear position negociating and compromising in Good faith if it makes you feel happier. Just answer me this Does it matter who mans the border if the troubles start again ?