UK/RoI CTA
The CTA only applies to UK and Irish citizens. Everyone else has always been required to identify themselves to Irish immigration even if arriving from the UK. It's all academic anyway now for a couple of reasons. Firstly, as noted, Ryanair only accepts passports as photo ID (or State ID cards, not DLs). Secondly, all passports now get checked at Dublin Airport, as there is no segregation of UK/Irish domestic and other incoming flights.
It is also worth noting that the UK is implementing an Advance Passenger Information system (in 2010 I think), whereby anyone flying into the UK will have to pre-supply basic info such as passport number, DoB etc (the same as the US, Spain etc). This will apply to entrants from Ireland as well as everyone else meaning, effectively, the end of the CTA.