I was travelling the past month from Dublin to Nice and on my way back they wouldn't let me board the plane because I "just" had an Italian national ID and they wanted to see an actual passport. Been traveling for more than 10 years within the EU and never had that happen to me before. After about ~10-15 minutes of arguing with border control that you don't need a passport to travel within the EU (I don't even own one) was granted access to the plane with a scoff and a "just get your damn passport next time and don't waste our time" line.
Just saying that it very strongly depends on the person at border control at the airport and a lot of different circumstances, you can get pretty tight checks from EU to EU too.
Dublin isn't in Schengen, so you actually do need a passport to travel between there and a Schengen country. I'm Irish, and can confirm that this baffles citizens of most other EU nations.
That's not true. Schengen is about lack of border controls. Being able to travel without a passport (using only national ID) is a EU thing. As a Polish citizen, I can travel to UK or Ireland without a passport, but I still need to go through a border check. Going to e.g. Italy I don't go through any checks.
You don't need a passport to travel to Dublin from mainland EU. You just need an EU ID card. The fact that it's not in Schengen just means that there are a bit tighter "passport" checks and controls and usually it takes a bit longer after you land to reach the exit. That's my experience with it at least, as somebody who's lived here for almost 2 years and been traveling all over Europe.
I haven't traveled Dublin/Nice but I haven't seen any border control at an Airport for Schengen/Schengen flights for a long long time (or car/train travel for that matter). Airlines sometimes check ID cards though and those people might not be up-to-date with regulations.
Just saying that it very strongly depends on the person at border control at the airport and a lot of different circumstances, you can get pretty tight checks from EU to EU too.