I haven't seen any airline make any comment on which routes they may choose to use their aircraft on.
At the moment, of the airlines operating TA out of Shannon, United and American have 737 Max orders and American have A320 NEO orders.
What could potentially be interesting is that SouthWest have ordered the 737 Max and JetBlue the A320 NEO. If either of them decided to go TA, it could shake up the whole market.