Wednesday's Irish Times reports that the UK-Republic of Ireland Common Travel Area -- the ability to travel without a passport between the two jurisdictions -- is on its last legs. With the UK planning to bring in electronic border controls i.e. an instant check of all arriving passengers against various databases, anyone arriving from the Republic will need a passport just like from anywhere else.
Now the common travel area has been dying a slow death, as (speaking from the inbound direction), immigration at Dublin airport has been checking everyone's passport for a long time now. Nevertheless, at least one glaring loophole will remain --
It does not appear that the British intend to apply the e-border to the land border between the Republic and the North, as that would be impossible to police.
So what about "bad guys" entering the islands through the Republic and then heading north to enter Britain via unregulated travel from Northern Ireland? There are only two options. Either the Republic and the UK have to agree completely harmonised entry controls, or travellers from Northern Ireland will have to be checked, with a passport, like those from the Republic when they enter Britain. It'll be interesting to see how that choice is determined.