We were worried about missing our bus due to possible inbound flight delay, so rather than book online we waited until arriving Dublin to buy our return ticket to Belfast. We were not told that the tickets we bought would only be valid after all passengers with online bookings were aboard. We did manage to board on the last 2 seats. On the return journey we found that all busses were already fully booked and had to make last minute travel plans back to Dublin Airport. We were never warned that the tickets we bought at the airport were potentially useless. Very disappointing.