I know that there have been answers to similar questions in the context of Brexit, and the consensus seems to be that it's not possible. But in the question about why the US-Canada border is soft at least in some places, the answer says that the border doesn't need to be hard, because the US and Canada simply didn't make it a priority. The answerer doesn't seem to see the lack of a customs union to be a problem at all.
Now we can be pretty sure that neither the UK nor Ireland would make a hard border a priority. On the contrary, it's very likely that it would be a priority for the both sides to avoid having a hard border even in the case of no-deal Brexit; nobody wants the return of The Troubles.
Does it mean that a soft border without a customs union is perfectly possible, but the EU simply doesn't want it, and uses its strong negotiating position to avoid such an outcome?
PS. I know that it isn't the case that the whole Canadian border is as soft as in the linked question. But the smugglers will obviously use the softest parts of the border for illegal activities, so that doesn't really change my points.