I read that there are busses going back to Dublin, that our guide can help arrange that. But I'd like to know well before that how it works, so that we can plan flights. Is it possible to learn more before we go?
To fill in a few missing details, the OP is asking about transportation from Belfast to Dublin. I think they are wondering about how to get to the airport vs. the city center.
Here is a link that may be helpful.
https://www.dublinexpress.ie/dublin-airport/belfast-to-dublin-airport?
Also, here is link to a you tube video that shows taking an intercity bus from Belfast
https://www.youtube.com/watch?v=G5B84jOudNM
The video is of a Translink bus- which is the much older competitor of Dublin Express- Translink are the state transportation company of Northern Ireland.
Dublin Aircoach are the 3rd company on this heavily competed for route.
Our RS tour ended at Hotel Europa/Belfast, and the Europa bus center/Glengall St is actually right next to the hotel, so it wasn't like it involved a lot of arranging. I think the tour guide asked us while we were in Belfast how many were needing to go back to Dublin on the bus, and then told everyone what time to be at the bus station. (Hopefully this sort of incomplete info will get you started with trying to look at links for buses departing from there; assuming your tour is also staying at Hotel Europa, which I think most RS Ireland tours do.)
We had prebooked a train back to Dublin. We had a Black Cab tour pick us up at the hotel for a tour then had them drop us off at the train station to catch our train back to Dublin. But we were staying an extra night in Dublin, so not trying to catch a train the same day as returning to the airport.
After the final tour breakfast I did a little more Belfast sightseeing, then got the Aircoach from next to the Europa Hotel in early afternoon, a two-hour ride to DUB, then overnight before a morning flight home. I got the ticket a few days earlier on their website. You have a link to the schedule, I think they're more or less hourly, but it would probably be wise to reserve a seat rather than just show up. The Translink service should be fine too.
Your best flight choice would be for the morning after you leave Belfast, rather than late the same day, just in case of a delay. There are two hotels at DUB itself, and several more around nearby Swords with shuttles. It's a great tour, hope you enjoy it.
Thanks everyone! I've got a better idea now, and as I was planning on a tour to Bru na Boinne before the start, we can return to Dublin and do the tour at the end instead, and wind up in Dublin for the morning flight.
We took the Aircoach right outside the Europa Hotel. We made our reservations about 7 days into our tour. The Aircoach let us off at the airport. We had reservations at the Raddison Blu for the night. The hotel was so close to the airport we walked to it.