Spirit operates a high density model, no?

What's the range of a heavily loaded 319 (let alone a 321!) off a 6,600 ft runway (Kerry) or a 7,000 ft runway (Cork) flying west?

A stop in Bangor or Gander is almost certain to be needed, not to mention some ETOPS.

There's a reason 757's do these routes, not Airbus NB's - performance.