So much for Aer Lingus market research, or perhaps for what will be their pricing policy under the new leadership. I've flown SNN -MAN regularly in recent years on FR and they had no problem getting 75% and greater loads on the route. FR won't, of course, re-instate MAN in a hurry as they've fallen out with them as well.
I don't think there was a problem with load factors on Shannon-Manchester, I think the issue was yield. Ryanair had to heavily discount the fares in order to fill the planes to the point where the route was loss making (it also affected loads and yield on Shannon-Liverpool). Aer Lingus are obviously aware of this, and would seem to have done enough research to know that they couldn't operate an A320 profitably on the route (i.e. they couldn't fill an A320 at the fares that would make the service profitable), and that an ATR72 operation would be more profitable.
But who knows? However, if they do launch the route with an ATR72, and it's as profitable as Shannonites claim, then I'm sure they'll upgrade it to an A320.