Most people would say the nightlife is the main draw -- pubs with live music and Guinness. The Cobblestone or So an overnight would be what most people would be drawn to. The airport doesn't have good transit -- there are buses and express buses for extra money -- but it's also not far so taxis are not prohibitive to/from the airport depending on your budget and time pressure. You don't have to stay at an airport hotel and it'll cut your night short to have to head up the night before.

There is a "free" walking tour during the day which meets at 11am and takes you to all the typical sights, I usually recommend that for first-time visitors. Tips are expected, 5 euros or more if you feel it's worth it. https://www.neweuropetours.eu/dublin-walking-tours/

State Museums are free so it's easy enough to drop in, see a few things, move on if you're not interested, etc. The Natural History Museum has "Bog Bodies" which are really interesting if a bit macabre.

The DAA lounges are worth using but not worth scheduling extra time around. They serve a pretty good selection of hot and cold food and some drinks but they're not big and don't have any showers or anything like that. The US lounge is a bit better and also has nicer views but it's still not worth scheduling extra time for -- though you will inevitably because you have to allow extra time for US security and preclearance but it's almost always pretty quick anyways.

The busiest times in the airport are surprisingly early in the morning so don't assume security will be quick at 6am or something. The US flights all leave around 9-11am so mid-morning can be a bit slower for US preclearance. There's really *nothing* (aside from the lounge) post-preclearance so don't go through more than 2h before your flight expecting to do much on the other side.