We are going on the 14 day trip in September ending in Belfast. Our ticket home leaves from Dublin at 9:55am. What is the best way to get from Belfast to Dublin Airport and what time should we leave to have a good cushion in case of unknowns?
You should leave the night before your flight and stay in Dublin to have a safe cushion. Otherwise map your time backwards from when your airline requires you to be at the airport prior to your flight. There is an express bus with frequent service that takes about 2h from Belfast straight to the Dublin airport. The cost is 10€ per person http://www.aircoach.ie/
Assuming your flight is nonstop from Dublin to the US, you really need to be at DUB three hours before your departure. That's because you go through US customs and immigration in DUB; your arrival in the US is like a domestic arrival. That's great on the arriving end, but it means you have to allow enough time before departure for the formalities - unless you don't mind missing your flight.
Short version: you need to be at DUB around 7 AM, and there's no way to do that besides driving in the middle of the night from Belfast, or getting to Dublin the night before.
There are direct buses from Belfast that go to DUB airport. I for sure would never get to an airport 3 hours ahead of time especially if you aren't checking bags and can get a boarding pass ahead of time. . I would think 2 hours is ample. However, this comes down to a personal decision and your comfort level.
There is a 24 hour bus service from Belfast to Dublin city centre via Dublin airport. Buses leave every hour on the hour, so just time your departure from Belfast to match the time you want to be in Dublin airport at.