Can imagine that would have been entertaining for my late father with a city of birth in India, while being ethnically British (and indeed a British Citizen at birth)... Added to which he'd have been entitled to an Irish passport under the grandparent rule . Guess it all goes to show that while the CTA rules sound fine on treaty paper, the reality is a bit of an admin nightmare where the only sensible solution is that above of "you don't need a passport, except for the purpose of proving you don't need a passport".

Now, don't get me started on the subject that (with significant hindsight) I really wish he'd exerted that right to Irish citizenship before I was born...