Dublin Express (or any other bus) to Belfast

There are three companies run the route-
Dublin Express is part of National Express in England (and Alsa in Spain), and run hourly with the odd gap.
AirCoach also run hourly with the odd gap (their service goes on to Derry via Belfast International Airport)
Translink (the National bus operator of Northern Ireland) also run every hour (every half hour from Dublin Airport). Translink would always be my choice simply because they were the original company on the route.

And all 3 services run 24/7, so you could have a really late night in Belfast and catch a bus back as late as 1am, or even 3am.

So there is a lot of capacity.

The primary purpose of these services is to serve the Airport so they are intended to be essentially turn up and go. Just walk off a flight and onto a bus is how they are meant to work.

At least on Translink you can buy on line until 60 minutes before departure and they never seem to be sold out. So a few days beforehand would be fine, even pretty much on the day.

And another option is to take the train.

And another option is to take the train.

The reason for this question is that there is no through rail service due to engineering work at Belfast- so it is a part rail replacement bus.

The reason for this question is that there is no through rail service
due to engineering work at Belfast- so it is a part rail replacement
bus.

In that case the train might not be a great idea! (I hadn't seen the OP's other thread.)

We used Dublin Express for that trip at the end of May 2024 and it worked out great. Our Belfast hotel concierge recommended it and said he used it all the time as he flew often to the US. We purchased tickets the day before our trip and that was more than sufficient.