I'm not sure I can see how that would work for either Shannon or Stobart.
From Shannon's side, Stobart have shown that they will pull out whenever pressure is applied.
For Stobart, they've seen time and again that Ryanair will target them.
There should be enough airlines at Birmingham that there are other options.
Or perhaps they could talk to VLM, as their new services from Waterford don't seem to heavily utilize the aircraft, with only 3 short-hops a day, leaving plenty of scope for at least one other UK-Ireland service.