Take Belfast bus to Sword, or Belfast bus to Dublin airport for last night stay

For a noon departure directly to the US you should be at the airport 3 hours ahead because you pre-clear US immigration in Ireland. So for a noon flight you need to be at the airport by 9.00AM. I'd stay in Belfast and take an early evening bus to the airport and stay at an airport hotel the night before your departure.

None of the several Belfast to Dublin bus routes stop in Swords but there are 2 local bus routes and a local airport shuttle - the #741 run by Ashbourne Connectt- which make the run in 15 minutes or less.
The #741 runs every half hour from 4am to 7am and stops at the Travelodge and the Premier Inn. So for all intents and purposes they are airport hotels.
Local bus #19 runs to the airport 24/7.
There is even a Castle in Swords open daily from 9.30 to 4.
So personally I think Swords is a reasonable choice.

Most of the Dublin airport hotels operate their own free shuttle service to and from the airport. Just confirm when you make the reservation.

I stayed at the Carlton Hotel a few years ago and the shuttle was easy. You just have to go to the shuttle pickup area.

Thank for your recommendations. While in Ireland we want to see as much as we can, and appreciate your opinions. Your bus info really helped.

Premier Inn at Swords is about 5 minutes from the airport. I would choose option 2

Thank you Frank for the information on the Carlton. After the Ireland in 14 days I'm flying from Dublin to Rome on an early morning flight and staying at the Carlton. It is great to hear that the shuttle is easy. Mahalo.

It was my impression that Swords is basically the area around Dublin airport. We stayed at Premier Inn near the airport (Swords) and they have a shuttle to and from the airport (costs a couple euros per person). As others have said there is also a local bus stop around the corner from the PI.
If you’re interested in staying at the Premier Inn I think it would be easiest to take the Aircoach from Belfast to Dublin Airport then get the PI shuttle if you’re not planning to go into Dublin at all. The area around the PI had a couple of casual restaurants and fast food within walking distance (nothing special or particularly characteristic but food was available) as well as a restaurant inside the hotel that we didn’t try, but seemed fine. Convenient enough for an overnight stay before your flight, for sure.

I have a follow up question. ( B) We decided to stay in Belfast and take an afternoon bus to Dublin airport and stay by airport for a morning departure. We have read there are two bus lines that leave close from Europa Hotel in Belfast to Dublin airport. Aircoach Express #705X and translink #X1. We found this info in RS book. We wanted to prebook online these tickets. What would you recommend? We will then take a shuttle to either PI or Carlton Hotel. Thank you again for your help.

You must have an old version of Rick's book as Dublin Express #785 also runs the route and have for 12 or 18 months now. They are the Irish arm of the English National Express.
As far as I personally am concerned Translink is the only company I would entertain using.
They are the incumbent operator, they run from inside Grand Central Bus Station (not a street side stop) and are the cheapest at £9 booked online. (£16 If you pay the driver).
You can on!y book on Translink 4 weeks ahead

Great! Thank you so much for your help. We'll buy our tickets 4 weeks out.