Belfast to Dublin airport

I wouldn’t take that chance. Not only will you be in a different city the night before your flight but in a different country. We had a very bad bus experience in Ireland. Not your route but we had an hour late bus & and hour and half late bus another time.

Do you want to take that chance before an international flight?

I would book the preceding service at 0520 to give myself a bit of leeway.

The bus is run by Dublin Express (a division of National Express) but it is cheaper booked with Dublin Express in Euro than with National Express in £.

Translink- the original Belfast to DUB bus operator (the National Bus Company of Northern Ireland) also run their X1 service at 5am, and they only charge £9 (half the price) when booked on line.- https://www.translink.co.uk/

They also leave from the comfort of Grand Central Bus Station as opposed to the random street side stop used by Dublin Express.

Their 0630 service also arrives at 0815.

I agree that taking the earlier bus is a good idea. I didnt know about the translink option so thank you for that! As always, I appreciate your help!!

We used both Dublin Express and Aircoach between Dublin and Belfast and both trips went well and on time without drama. So in theory your 6:20 bus would work. But I personally would be nervous not being in the departure city the night before an international flight. Also with the pre-clearance at DUB you want to give yourself plenty of time (3hrs is sufficient IMO but what if the 6:20 bus is delayed in traffic or something?). I would choose an evening bus back to Dublin and stay near the airport and have a leisurely morning. But that’s just me.