Best one Day Visit East Coast

No and not an easy place to get to by public transport.

Where does the tour start?

@ Claudia The tour starts in Dublin and will be arriving a day early.

If you look at google, it’s about a 5.5 hour trip to Waterford via public transit from Belfast. If the tour ends in the morning you could conceivably be in Waterford in time for dinner. But then you’d have to leave at zero dark 30 to get to the Dublin airport by 11:00 am, not noon. I assume you are flying to the US, which means clearing immigration in Dublin. And you really do need to allow 3 hours. So if I were you I would go back to Dublin and do the things the tour skipped: the archaeology museum or Chester Beaty library for example, or take the bus to Malahide or the train to Dun Laoghaire.

@Trotter Thank you for the suggestion. Yes 3 hours for the airport. I miss typed.

Personally, I'd lose sleep if I had to worry about getting to Dublin airport from Belfast on the morning of my flight out. To save yourself a bit of stress on getaway day why not return to Dublin after your tour ends, then use your discretionary time to explore more of Dublin and its environs than what's covered on the tour?
A day trip up to Newgrange and the Boyne Valley is one thought, or south to Glendalough and/or Powerscourt. There are plenty of tour companies that do one day excursions to either of those. If you're "toured out" and would like to just strike out on your own you could hop the train down to Kilkenny for the day - nice little town, pretty castle, plenty of photo ops, and only an hour and a half from Dublin by rail.
Staying over in Dublin the night before would allow you to sleep in, have a leisurely breakfast, maybe explore a little more of the city if you're up early, then catch the quick, efficient and cheap Airlink bus to the airport whenever you're ready.

@Robert. I think you are correct about loosing sleep. I am thinking of stay at a hotel by the airport. Kilkenny sounds great but getting there from Belfast with a stop off at the airport hotel to dump the luggage is around 5 hours. It is almost 3 hours for Kilkenny to Dublin airport. Another thought stay at Dublin Castle.

Didn't realize that you were trying to do this in a single day. In that case I agree that it's probably best to just stick to Dublin.
There's really no need to isolate yourself out at the airport the night before departure. We've found that staying in central Dublin works just as well - the Airlink bus really is a great option and will get you from your hotel to the terminal in about 20 minutes.
Taxi's are pretty cheap too (around 30 Euros as I recall) and have the advantage of door to door service which is nice if you're horsing around a lot of luggage.

I can second the Airlink bus recommendations. Choose a hotel in Dublin near whatever interests you that your tour didn't cover, and check its location in relation to the bus route. Allow plenty of time in case the bus is late (Dublin has traffic jams like any other city) and your stress level will be low.

I wouldn't worry in the slightest about getting to the Dublin airport from Belfast or Dublin. There are frequent buses.

The real question is whether you want to spend your extra day in Dublin or Belfast? Choose which city you would like to spend more time in. I don't know how things will be scheduled in a Covid-19 or "post-Covid-19" world, but one activity we really enjoyed was going to see the Riverdance performance in Dublin.

I hope all works out for you to take this tour.