We don't have to show passports when we go to the Isle of Man or the Channel Islands, and they are not in the EU. The German island of Helgoland isn't in the EU.
There may be slight anomalies, but no major European country gets to be part of the Single Market and gets to set their own terms on freedom of movement. The examples you state are tiny little islands with populations in the thousands. It's in no way comparable.
And anyway, the UK's own position makes this untenable. They can't simultaneously 'take control of their borders' but not that border, as has already been debated extensively on this thread. The UK are asking the EU to do some ridiculous magic thinking for them. It's dishonest, as this Vincent Boland article states much better than I can.
The Brexiteers tell us at every opportunity that there must be “no hard border” in Ireland after Brexit. Yet the strategy they are maniacally pursuing, of a “cliff-edge”, or “hard”, or “no deal” Brexit, will create that very thing, with all the administrative, physical and political infrastructure that will be required to make it work. That is the political and legal reality of the UK’s decision to leave the EU single market and customs union, and not a plot by eurocrats to punish Britain. The argument by pro-Brexit advocates that if there is a hard border in Ireland after the UK leaves the EU, it will be the fault of Europe and not the UK, is dishonest.
Full article here, I suggest you read it to get a sense of the mood towards UK in Ireland. Not one soul thinks the EU is at fault here, trust me.
www.irishtimes.com/opinion/boris-johnson-might-find-his-irish-cake-hard-to-digest-1.3291764
Why all the fuss about NI? Given the history and the efforts given to the peace process, just leave it alone. Ireland is an island, it's hardly going to be a hotbed of smuggling either people or goods - it's not like a land border on the continent. There really isn't any need for customs posts or passport checks. It has no effect on the EU's external land borders elsewhere.
Well, you clearly don't know much about Northern Ireland if you think smuggling isn't going to be an issue. It has always been rife. In answer to your question though, the main thing that makes it different is it's divided population. One side wants close ties with ROI, the other wants close ties with GB. Now that UK are leaving the EU, that juggling act is going to be much more difficult to maintain. For example, the nationalist side would accept an open border with ROI, with a hard sea border. No way in hell are the DUP going to sign up to that. So herein lies your problem.
All that said, I do think it's outrageous that the referendum was called knowing that a requirement of the Good Friday agreement was that both Ireland and the UK were in the EU. Has anyone looked into legal action to see if Brexit can be stopped because it's in breach of a treaty?
It's not about EU membership, but as I understand it, a requirement of the GF agreement is no return to a hard border. Which is exactly what the tories are charging right towards if they crash out without a deal. You can understand Irish concerns around this, I hope. Certainly Sinn Fein have made lots of noise about the potential breach and I'm sure some legal minds are on it, but I'm not sure where they've got to.
Which takes me back to my point that the very least the UK could have done is sat down with the ROI government, long before the referendum and discussed the border issue in the event of a leave vote.