European Union leaders are threatening the United Kingdom of Great Britain and Northern Ireland (UK) with a "no deal" on Brexit due to their assessment of failure to make progress in key areas, including specifically the thorny problem of how to deal with border barriers between the Republic of Ireland and Northern Ireland, which forms a part of the UK (see here and here).
The problem apparently has two facets: One is the matter of trade and customs duties; the other is about the demand for free movement between residents of the two politically distinct areas, while not compromising Britain's right to control its borders — including of course migrants wishing to enter the UK.
I have little to say about trade and customs duties, but at the risk of sounding simplistic, the other area having to do with immigration inspections of intended entrants seems to me to be eminently solvable, and this is because the Republic of Ireland and Northern Ireland occupy the same island territory, whereas the larger part of the remainder of the UK sits on a different, larger island, separated by the Irish Sea.
UK and Irish authorities could simply allow the same kind of mutual ingress and egress that is now enjoyed by their citizens and residents in Ireland and Northern Ireland. However, the UK could then invert its typical immigration inspection model, and employ exit controls over all air and sea departures heading for any portion of the rest of the UK, from England to Scotland to Wales, and even the Isles of Man and Anglesey. This would assure that only legitimate and bona fide UK citizens or legally authorized foreigners actually are able to gain access to the rest of British territory.
As for maritime patrols in the Irish sea, one assumes that these go on regularly anyway, for obvious reasons having to do with interdiction of drugs and contraband. Expecting those British naval, coast guard, or customs and excise personnel to also keep an eye out for intended illegal migrants is no great additional duty, if indeed they aren't already doing that anyway.