Surely that can work the other way though? Any of us can fly or get the ferry to Belfast then just stroll across the border and into the EU with no checks. I would imagine that people living in the Northern Ireland/Republic of Ireland border go in and out of the EU dozens of times a week just when they're driving around in their daily lives. And then if we wanted to avoid all the biometric checks for non-EU people that have just come in we could fly to our EU destination from Dublin airport. As it would be EU to EU we could skip all the fingerprint checks that we would have to go through if we were flying from the UK (non EU).
Just to add to what others have said:
The UK and Ireland (and the Isle of Man/Channel Islands) have a Common Travel Area, meaning that for the purposes of the law, neither British nor Irish citizens are 'foreigners' in the eyes of the law in any of the jurisdictions covered by it. You can travel freely as a British/Irish citizen between the two countries and Crown Dependencies, although there are some specifics such as Ireland requiring some form of photo identification (although a bus pass is enough for British/Irish citizens) when you arrive at a port, or the Isle of Man planning to introduce ID checks on entry.
So, while Ireland is part of the EU, British citizens have the right to stay there freely and without formalities based on a different set of laws. While the modern day law is based specifically on the Aliens (Exemption) Order, 1999 (S.I. No. 97/1999) in Ireland which exempts British citizens from the Aliens Act of 1935, the initial exemption was based on the Aliens Act of 1935 itself, which gave the government the power to decide which citizens were exempt from the act. As the Act defines an alien as 'anyone who isn't an Irish citizen', what happened in Irish legal theory was that British citizens were simply not considered aliens in Ireland.
The CTA itself is based on a very odd patchwork of legislation, but it came about because while Ireland established citizenship of the Irish Free State in 1922, they also didn't introduce any laws defining what an alien was until 1935. As a result, there was no need to introduce identity/immigration control in 1922, and then the border had become well established by 1935 as one that didn't require identity controls. There were also other practical considerations, such as Ireland not recognising the UK's jurisdiction in Northern Ireland, and establishing identity controls would have undermined that.
However, Ireland aren't in Schengen, and anyone travelling from Ireland to Schengen is only exempt from customs control, not police/identity control. EU citizens arriving in Ireland go through the same identity controls as anyone else as well. All the EES checks are carried out exactly in the same way, that is, EU citizens/residents are exempt while everyone else has to go through them on arrival at a Schengen airport or ferry port.
What is the position for Irish (Republic) citizens travelling to the Schengen area? My assumption is that they have to go through Schengen 'for EU citizens' entry procedures (does this now include the new EES ones?). And that UK citizens - like any other third county ones - entering the Schengen area from Ireland will have to go through standard Schengen 'non EU' entry procedures, in the same way as coming from any other non-Schengen country.
They go through police/identity controls for EU citizens, just as if they were arriving from the USA or anywhere else. There's no need for EES registration and so on, as they have freedom of movement in the EU. It's exactly this 'backdoor' that makes a mockery of Brexit, because the Irish immigration authorities can't and won't stop EU citizens from entering Ireland. With the open border, EU citizens can effectively walk into the UK and stay indefinitely as long as they don't draw attention to themselves. On the other hand, Ireland and the UK do cooperate when it comes to non-EU citizens, and it's quite routine for both countries to deny people entry if they think that immigration regulations will be broken in the other jurisdictions.
This is why an identity card solves many problems, because it abolishes all the insecure forms of identity currently used. As it stands, someone with a neutral accent from somewhere like The Netherlands can easily get away with staying in the UK indefinitely with a fake birth certificate, and there's certainly plenty of them in the UK who pass for locals.
Fair enough, I hadn't realised that. As long as they stay in Ireland then, it would still be quite possible for a UK national to enter the EU via the Irish land border with none of the usual EU entry checks and, potentially, stay there for some time without encountering a situation where they got prompted for ID.
Ireland has their own immigration regime, separate to Schengen. They participate in some aspects of Schengen (just as the UK did before Brexit), but in general, Ireland is responsible for her own immigration affairs, as is Cyprus. So, as mentioned above, British citizens are not considered aliens in Ireland, and they can stay as long as they want without doing anything. I don't even think Ireland has any formal registration process for British citizens, just as the UK doesn't have any for Irish citizens.
and even now is just a memorandum of understanding between the relevant governments.
From what I understand of the legal position, it's that the CTA is already defined through various laws, so there was no real need to pass new laws. The UK did amend some laws to make it absolutely crystal clear that Irish citizens are exempt from any/all immigration control, while other laws already covered issues such as Irish citizens being automatically considered 'settled'. There is an interesting legal point in that any child born in the UK to an Irish citizen is automatically a British citizen as a result. The UK also made it even easier this year for Irish citizens to obtain British citizenship by removing the requirement for them to pass the "Life in the UK" test (which was frankly ridiculous to begin with) and also removing the requirement for them to prove their knowledge of English (which was a formality, but still required them to show that they had obtained some qualification in English at school).
The Ireland Act of 1949 explicitly says that the Republic of Ireland is not a foreign country for the purposes of any UK law, so Irish citizens can use their birth certificates to prove their right to work. Now, how many of us would be able to identify an authentic Irish birth certificate from a fake one?
But again: without ID cards, how do we know that someone turning up with an American accent and an Irish birth certificate is, or isn't entitled to work? The documentation says that Brad was born in Dublin to Irish parents. Yes, Border Force can check with the Irish authorities as to whether it's a legitimate document, but in practice, it's very unlikely that they're going to get caught. With a digital ID card, we can then require Irish citizens to either obtain the British digital ID card, or to use a document confirming citizenship from Ireland.
This is before we even discuss the headache of the Northern Irish, many of whom hold Irish passports and yet are considered British citizens by the UK through nothing more than their birth certificates. They often hold Irish and not British passports, so what then?
It's actually astonishing just how many loopholes there are, and it really makes you wonder who might be behind any push to block identity cards that clearly identify someone as a British citizen with the right to live/work in the UK.