Another data point to share. My family of four recently went to Ireland. Apparently, my daughter's AA account has had the wrong birth year, so it didn't allow us to check in online because it didn't match her passport. That had me a little nervous, but nobody questioned it when checking docs at the airport.

On the way back, we checked in in DUB at the counter and checked our bags. After doing so a MCE seat opened up and I moved myself to it. (I was on the aisle across from the 3 seats for the rest of the fam & this was just a row ahead). I assumed I could just get my mobile boarding pass at that point. I was incorrect. The app told me that mobile boarding passes weren't allowed in this airport. We were in the security line when I realized this. I didn't want to go back downstairs to the counter as that line was a zoo because a different flight canceled. I took my chances and we were allowed through security on my original pass. I thought it might be invalid because of the seat switch. This got us through both levels of security and then customs. Then I was able to get my correct boarding pass at the gate about 20 minutes prior to boarding.