And all this is to prevent stuff like terrorism, right?

So, we do it now after the "Troubles" have largely ended?

Yeah, right.

The excuse is that unless our e-Borders with Ireland are as e as our borders with everywhere else, then illegal immigrants and Islamist fanatics will first go to Ireland and then sneak in to the British mainland. "Paranoiac fantasy" is probably not strong enough for this ludicrous argument. But then, the current generation of our political masters live entirely behind behind fortifications and screening procedures, and get their information about the world from alarming briefings from other regulatory fetishists.

Readers might wish to suggest what the e in e-Borders really stands for. Other than eejit.

Back in the days when the British immigration authorities would check your passport on leaving Britain as well as arriving, Australian backpackers and the like always understood that the way to leave the UK without either getting inconvenient questions from a British official or an exit stamp with an inconvenient date in it in your passport that could cause problems later was to get a ferry to Ireland and fly out of Dublin. The British actually stopped looking at and stamping passports of people exiting a few years back, so this is less of an issue these days. Similarly, if you had (say) overstayed a visa in the past and wanted to return to the UK, then going via Dublin was a possibility there, too. In terms of anything serious, these things didn't work, as the Irish were on the lookout for criminals and terrorists to about the same extent as were the British.

So, yes, Ireland has always been a bit of a leak to the British immigration system. I have generally considered this a good thing.