The problem for Aer Lingus is that they don't have aircraft based in Manchester or Knock. That doesn't rule it out; some Manchester-Cork services have operated as W-patterns out of Dublin in the past. However, it does make it less likely.
Until Aer Lingus open another overseas base (and I don't think that will be soon considering the state of the company's finances), I'd be surprised if they offered more Knock services.
If they were to look at W-patterns, a more interesting option might be to offer a connection to an international hub. At the moment, that service is just not available in the north-west and there must be some level of market for it.