Logistically that would be the simplist solution, easier to "quietly" carry out customs checks on trucks/containers that are heading towards a port. But as you say "that will inflame Unionist opinion".
Many years ago, I remember a young unionist lad saying in a TV interview that "they were more English than the English themselves", I was just a kid myself at the time but I still remember my thought, "but you are Irish"
You will be only too aware of the issue of flags and marches in N.I.
The Unionist are not in a happy place with demographic changes due to birth rates and so many young unionist moving to GB they are seeing their majority (and Britishness) slowly errorded.
Recent figures show there are around 50% more students from a Catholic/nationalist background going into higher education here as there are from a Protestant/unionist background
http://www.judecollins.com/2016/01/a...er-university/
It is terrible for the Unionist tradition to see their bright young people deciding they have had enough and leaving.
In the now suspended Stormont government the DUP had one one member more than Sinn Fein. When (if) Stormont is restablished, it is only a matter of time before there is a Sinn Fein first minister.
Before brexit N.I had fallen out of national news, but it is still a very troubled place and will be for many years to come.
Don't blame Leo Varadkar for trying to find a way of lessening the tensions up there.