I know it isn't legally required -- the CTA and all -- but some people have, in the past, been told that they had to have a passport on what was a domestic flight (within Great Britain). Goodness only knows why this was: perhaps it could be required by some or other obscure regulation and the airline had unconnected reasons for not wanting that specific** passenger on board and this was the easiest way to achieve that end. But whatever the case, I'd rather not find out at the airport.
Oh, and come 2019, some deal may have been done regarding easing travel between the North and the South and/or you may have the right to be an Irish (and thus EU) citizen, but I live in England and my most recent Irish ancestor (from Downpatrick) was my... er... great-great-great-great-great-grandfather, so I'm pretty sure that route isn't available to me. (And I really do need to get a passport anyway.)
** - I've mentioned this elsewhere (probably in WA, so can't check), but in the 1990s, a colleague of mine told me that he was often called aside by security folk at airports when travelling within the UK. I assumed he was pulling my leg... until I was on a flight with him (on business) from Southampton to Manchester and, when we landed, he was called aside.
It couldn't have been based purely on appearance: he was blond, while my hair was still red back then (so I perhaps looked more stereotypically "Irish"*** than he).
*** - And it isn't as if a person's appearance is not sometimes used to "determine" things like their "potentially hostile intent", even today.
