A key clause of the Belfast Agreement - of high importance to Irish nationalists and republicans - was the future granting of both Irish and British citizenship to anyone born in Northern Ireland.
The subsequent enabling legislation in Irish nationality law has several clauses, including that any child born "on the island of Ireland on or after 1 January 2005 ... is entitled to be an Irish citizen if at least one of his parents or his or her parents is ... a British citizen". Other parental criteria for birth-citizenship are of course Irish citizenship and permanent legal residence without time-limit within either the Republic or Northern Ireland.
Little remarked upon after the Belfast Agreement was the fact that Northern Ireland had become a de jure multinational territory with a written Irish-British constitutional treaty text lodged at the UN - in dramatic contrast to Great Britain, with its unwritten constitution and single birth-nationality. Northern Ireland is possibly the only such jurisdiction in the world where all births are now automatically bi-national.
The aforementioned "British-citizen clause" was clearly a quasi-symbolic gesture of national exclusiveness by the Dail directed toward the indigenous Northern Irish of unionist outlook. In 2005, this clause had no impact on the question of possessing an EU passport. But that has changed dramatically with Brexit.
Since last June, incredibly, this massive loophole has not even been remarked upon publicly by politicians or the media in either Ireland or Britain. Month after month, one might have expected that this major issue would be flagged up with the seriousness it merited, especially apropos the post-Brexit future - but nothing at all, after 10 months.
The general oversight no doubt flows from the fact that "British Brexit Birth Tourism to Northern Ireland" - as it might be called - is rooted in Irish law ("Irish Nationality and Citizenship Act," 2004), rather than anywhere in British law. According to the husband of a Belfast midwife known to this writer, it is already happening.
Thus, in theory, any of the approximately 750,000-800,000 babies born annually in the UK could for generations be entitled to an Irish/EU passport if the birth-venue is switched to Northern Ireland. That this is Irish law has been confirmed by Bernard Ryan, Professor of Migration Studies, Leicester University, as well as the Republic's Department of Justice.
In effect, any British citizen in Great Britain (as mother or couple) who now wants an EU passport for their new-born can travel to Northern Ireland for the birth. The result is a huge unintended and overlooked "Remainer" loophole flowing from the complete absence of consideration of Irish impacts during the mainstream referendum campaign in GB.
Such "British Brexit birth tourism" will be of extreme concern to the EU especially. It raises the clear long-term possibility that, even after Brexit, there is a permanent massive route to EU citizenship for new generations of British births, so long as they occur in Northern Ireland - whereas the reverse cat-flap would not operate. And this in addition to the many British citizens already entitled to Irish passports via the traditional and well-known route of blood ancestry (estimated at up to five million).
Nor is this simply a rich family's option before the ban on flying when pregnant. Cheap Northern Irish ferries run from close-by Scotland and Northern England - affordable to any working class couple. And whereas birth-tourism to the Republic - whether from Nigeria or England - could be legally controlled by the Republic in the aftermath of Brexit, the free movement of British citizens across the UK cannot be constrained.
All changes to the Belfast Agreement require, both legally and practically, consent from both the "unionist" and "nationalist" political traditions. It is highly unlikely that nationalists and republicans will concede any diminution of the unusual bi-national nature of the North. They will exert their de facto veto - and "birth tourism" could well become most intractable and insoluble of all the unseasonable Brexit impacts on Ireland.
Implausible? Recall the first ludicrous under-estimates of Eastern immigration to Britain in the aftermath of accession to the European Union by the 10 Baltic, Central European and Mediterranean states in 2004. Where a citizenship right or loophole exists, it will be used - and used hugely. Especially now.
Amazing oversight! Astonishing problem!