The fleet isn’t owned by Aer Lingus. They are leased from a few different places. Nordic Aviation Capital, Propius (which is connected to Stobart) and GECAS.
Isn’t an EU AOC required for the ROI routes? While a U.K. one is required for the Belfast routes.
There are plenty of carriers able to do either. The question is price and what EI’s plan actually is.
It may be easier to just use an A320 on limited frequencies (say 2-3 weekly rather than per day). Not like the market is booming at the moment.