Tour ends in Belfast, I need to catch flight home from Dublin airport

I did the Best of Ireland when it ended back in Dublin so don't have experience getting back from Belfast but here's what the information on the Best of Ireland transportation tab recommends:

"Flights

Our tours don't include airfare, so your flight arrangements are completely up to you.

Most tour members arrange to fly in and out of Dublin (DUB). If you prefer to fly out of Belfast (BFS), select "multicity" on flight-booking websites. The coach from Belfast direct to Dublin Airport takes about two hours and departs hourly 24 hours a day. Your guide can help you book it when you arrive."

Belfast is a lovely city. Much smaller than Dublin. A very walkable city. If you have limited time, as you seem to have, a visit to the Titanic Museum and related sites is well worth your time. That’s assuming you would be interested in Titanic and its ever-so-brief life. Another possibility would be a Black Cab tour, seeing the sites of the Troubles. Drivers can be chatty and very knowledgeable.

AirCoach would probably be the best way from Belfast to Dublin Airport. A train would take you into Dublin city center and then you would have to get back out to the airport. AirCoach runs regular service from Belfast directly to Dublin Airport. Large comfortable coaches, plenty of space for luggage. Dublin Airport can be congested and confusing. And flying from Dublin to USA, you do immigration and customs there, not back home in USA. Convenient for your arrival in USA, like coming in on a domestic flight. But it means you need to allow extra time at the Dublin Airport for that added processing. I think being at the airport three hours prior to departure would not be excessive, but prudent.

Translink - the Northern Ireland national operator also runs their competing X1 service direct to Dublin Airport- https://www.translink.co.uk/

If this is a RS tour it will include the Titanic sights and a bus ride through the formerly warring neighborhoods. On my last morning in Belfast I visited the Ulster Museum, which I thought was quite good, then took an afternoon Aircoach (Translink would work too) to Dublin Airport, where I spent the night before my morning flight. If your flight isn't until afternoon the day after the tour, you might want to go right into Dublin the morning the tour ends and have a partial day in the city.

I didn't take the train but the bus ride wasn't especially scenic. If you've been on a tour you've probably seen a lot of better scenery already.

I’ll be on this tour earlier in August and plan to take an early bus (two hours, direct) to the Belfast airport; my flight leaves at 11-ish. It will be a long travel day but the flights work better for me this way.