I think you need to take account that the Dublin route is a BA flight number but as I am aware is still operating under Bmi out of T1. The merger of the two airlines is a massive task and once the route transfers to BA you will then be able to check in online. Lots of challenges for everyone but as a customer you just expect and want it to work. Stick with it and as the merger progresses things will flow better. Things like self check in etc take a little bit of time to sort but I am sure BA are on it.