We just got back from our trip. We bought our rail ticket to Belfast online, about 2 weeks ahead. Collecting it at the station was quick and easy, I just entered the code into a kiosk and the tickets were printed. Since we bought online, our seats were reserved and labeled on the electronic banner above the seat. Available seats are marked in green. There were some seats left open as we traveled.
We applied for our UK ETA about a week ahead of travel, it was easy on the app. Once approved (response via email in a few minutes) you don't need to save any receipt/email/code - it's just electronically attached to your passport number. Since we were traveling from Dublin to Belfast, our passports were never checked. Now we're ready for our next trip to the UK!
On our return we took the bus from Belfast to Dublin airport, having booked our tickets about a month ahead, on Aircoach - we chose that line because it was the best timing (7:10am, stopping at terminal 2 first about 9:25). It was easy and smooth. The seats were fine, nothing luxurious. We were in the first group to board at about 7am when the bus pulled in but by the time we left the bus was completely full. It was a bit loud with everyone talking but I had my headphones on and listened to a book the whole way. All in all, an efficient, reasonably priced way to get directly to Dublin airport from Belfast Grand Central station.
