The biggest training issue is down to NAT operations, EI crews are all good with oceanic as they fly the oceanic route to avoid France to Spain and drop in on Iceland on charters from time to time, they also use CPDLC. So you will need training and hands on with NAT procedures.
ETOPS could be a factor as well, EI have extensive experience in this though
The A321 has new engines and also the winglets so it handles a little differently
I would guess a few flights will take place but that could just be a few trips out to Shannon for touch and go. I would put money on CDG if only for the reason the very first commercial flight of the A321 for EI was to CDG, the Minister for Transport forgot her passport which led to some fun.