You can pre-clear in Dublin for Chicago. I once flew Birmingham – Dublin – Chicago and was out the airport in Chicago before we were even scheduled to land. And clearing immigration in Chicago is normally pretty crappy.
Always found most trans-atlantic carriers much of a muchness. Although was on a crappy old plane with Aer Lingus and they had a fancy brochure showing you what you could watch on the in-seat entertainment if you weren’t on one of the crappy old planes (there were upgrading at the time). The ‘Here’s what you’re missing;’ bothered me more than if there was just no in-seat entertainment.
I think that they still fly out of Terminal 5 at O’Hare on the return leg, nothing beyond vending machines after security so don’t go through with plenty of time to spare thinking you’ll grab something to eat 😉