What I find interesting is the "Common Travel Area" - i.e. UK & ROI.
As a British Citizen I am entitled to travel to the ROI without a passport, but how do I prove I am a British Citizen without my passport? I had this discussion with the Gardai at Dublin who said "but you've arrived on FR - you need a passport for their terms of carriage" - fair enough. At Gatwick they were happy to see my boarding pass ex DUB to let me into the domestic arrivals channel - when I could have been a citizen of any country!