Smaller aircraft will be used for more transatlantic services, but the A321 isn't the machine for the job.
For a start, EI's current A321s are configured with more than just the 180 seats (212 out of memory). You'd end up flying with 25% of the seats blocked off, which isn't going to do much for keeping costs down.
Also, the distances are close to the maximum range. A bit of a headwind and you'll have a tech stop in Gander. That doesn't make it easy for scheduling or costs.
Finally, I'm not certain the runway lengths at Cork and Knock are OK at MTOW.
Like I said, smaller aircraft will be used, but it's more likely to be the A319 or a long range 737, neither of which Aer Lingus have.