airport to Dublin City center and reverse? Uber?

Not really an answer to your questions but in August we had our hotel arrange in advance a taxi from The Chancery (near Dublin Castle) to the airport on a weekday at 8:00 am for 42 euros (four adults with four pieces of carry on luggage).

We've always used a taxi for getting to and from the airport - doesn't cost that much more than the airport bus and we appreciate the convenience of the door to door service when we're toting luggage around.
Uber does appear to be an option. The rental car lot is located about a mile away from the terminal along Swords Road, so taxi or Uber would be the easier way to go.
I don't know where the Uber pickup point would be but I'm sure it's pretty straightforward.
I'd probably book ahead for a taxi back to the terminal on getaway day, especially if the flight is very early - you'll need to pass thru the pre-clearance facility for US Customs & Immigration as part of your departure procedure, which is convenient over all but which does require extra time upon arrival at the airport.

Thank you. Our return flight is at 8am. Would need to be there earlier than 6am? Was thinking leaving hotel at 5am would be okay? We are flying on a Sunday....

Others who have done this more recently may want to weigh in with their more recent experience, but the general advice for flights returning to the US is to be there at least 3 hours prior in order to have a comfortable cushion for making your way thru pre-clearance. Anything less than that is liable to be a little risky ... and stressful.
Edited to add: you may want to check with your airline to see what they recommend ... and to verify that their check-in desk will be open at 5 AM.

Thanks for the heads up regarding clearance in Dublin. Correction - flight is at 9am (sunday). That said, is the consensus that we should arrive by 6am?

Ride share programs like Uber and Lyft do not operate in Dublin. The app will work but will call you a taxi. Freenow will also call you a taxi. As mentioned previously, the car rental lot at DUB is remote. I don't think there's a taxi rank there. You could shuttle back to the airport and then grab a taxi from the terminal. For your return flight you should be at the terminal 3 hours ahead since you preclear US immigration at DUB. If your flight is very early you probably should ask the hotel to call a taxi for you. Otherwise, if there is a taxi rank near your hotel you could just get a taxi there. You do not hail taxis in Dublin like you might in NY or DC.

Here is the ground transportation page from the Dublin Airport website. You'll see that there are LOTS of ways to get to/from the airport. The last time we flew into Dublin, we took the pubic bus for like 5euro. It had luggage racks and upper-deck seating. It probably took a little longer, but we enjoyed the scenery, and it dropped us near Connolly Station, just over the Liffey from our accommodation.

Thanks...your link didn't post but I accessed the map. jockeying back to the terminal is probably not our best option. we are driving, not flying in....

Once you've dropped your rental car, if there's no taxi rank at the rental car place, you could use Uber--yes, it will call a taxi. But it's just a few clicks of your phone and you're all set.

To return to the airport for your 9am flight, I agree it's best to let your hotel prearrange so there are no surprises at 6am.

DO plan to get to the airport 3 hours before your flight. You may breeze through. But you may also get pulled aside for random screening, and sit for 45 minutes as your luggage is opened and each item inspected. That happened to us once.

Have a wonderful time!

Hello, we did exactly this scenario 2 weeks ago. The rental company ( Europcar) had a shuttle to the terminal where we caught an express bus . We were staying just south of Merrion Square so we had a bit of a walk to the hotel from where the bus let us off ( near Trinity) but it was certainly not a problem.The bus fare was 8€ each, pretty reasonable. Our flight home was also at 9 am, the hotel arranged for a taxi for 6 am, plenty of time as there was no traffic Sunday morning. We were through security in no time and had more than 2 hours before boarding (even after a quick shop). We are Canadian so we cleared customs in Toronto not Dublin so I can’t speak to how long that might be.

We've done the same thing. We returned the car and the rental company gave us a shuttle ride to the airport after a very short wait. Then it's just a walk across the street to the taxi queue. I used Freenow to get an early taxi to the airport. I'm never at the airport the recommended 3 hrs before a flight, but we travel with just carry-ons and do online checkin.

Irrelevant, but in Scotland, BritAir has data that allows them to make a reservation that connects a domestic flight arriving at Gatwick to an international flight departing from Heathrow. Sounds impossible, and I declined.