Returning a car then flying out of Dublin

We last rented from Enterprise, and they had a drop-off near the airport, but not actually at the airport. It was more convenient, and I think avoided the extra airport location fees. But don’t drop off inside the city - stay away from downtown streets and traffic.

We rented twice from Conns Ireland, a Hertz Franchise, https://www.connsirelandcarrental.com/. They have multiple offices including Dublin on the South Circular road, off the N7 giving easy access to the west and south, and the Dublin airport. No additional fee to rent from one location and drop at the other.

As Cyn notes, the rental car lot at the airport isn't at the terminal but rather about a half mile away along Swords Road, so if you're coming from Dublin you'll want to ensure that your taxi driver knows that's where you want to go. The advantage of an airport rental (and drop off) is that the concessions there open very early and close very late. They'll also have a larger selection of vehicles and a greater likelihood of getting exactly what you want ... including a vehicle with automatic transmission.
There are rental car offices on the outskirts of Dublin city which might be handy for departures to the south, including a couple along South Circular Road that others here recommend, and from which it'll be a straight shot out of town so as to minimize any time spent in city traffic. The downside is that those offices open late and close early which may not make them a good choice tor your purposes.
If you're heading towards Kerry or Dingle you might look at catching one of the two daily RyanAir flights between Dublin and the Kerry airport near Killarney. There are rental car outlets there that would be convenient for a few days in the area, with the option of then dropping the car back at the airport and catching a return flight to Dublin in order to be in place for your return flight home the next morning. Would save you a couple of long driving days.

We picked up and dropped off at the airport. After we dropped off the car we took the bus into Dublin for our last night, then took it back to the airport on departure. If your flight leaves in the afternoon, Trim is 40 minutes away. It’s a nice town to spend a night.