The Premier Inn near the airport is relatively inexpensive. The rooms are updated and clean and the a/c works well. There’s a shuttle ( €3) to the airport every half hour on the :15 and :45, and it’s a very short ride. We stayed there 3 weeks ago the night before our 9:30 a.m. flight. With having to do all the immigration and customs stuff before boarding your flight, you should allocate 3 hours at the airport, which in your case means being there by 7:00 a.m. We were super thankful for the extra half hour of sleep and not having to stress about morning traffic jams because we were already near the airport.
In our situation we had one final day in Dublin after returning by train at noon from Belfast. We pre-arranged to store our suitcases at a Stasher location near Connolly Station, enjoyed the day and an early dinner in Dublin, and then went to the hotel early evening.
I first heard about this particular hotel from others on this forum.