In a case like this, I assume the DL agent in Dublin would tag the bag only to ATL, right? You'd see the agent attach the ATL tag to your bag (not BHM), so you'd know that the bag isn't getting past Atlanta no matter what.

Of course, by the time you're handing over the bag to the Dublin DL agent, you'd also know whether you're going through a preclearance or not...

In any case, short-checking with an overnight connection is a fairly normal practice. I've actually had Delta do it for me in Atlanta on an outbound international itin. Even if DUB is doing full preclearance, I don't think a DL agent will argue with you if you tell him/her you need your bag for the 20 hours in Atlanta.