NOBODY on God's Earth is going to fly Ryanair to connect to Aer Lingus out of Dublin on long haul. The only thing worse than a normal short haul connection at the end of an overnight transatlantic flight would be if it were with Ryanair. In all seriousness, even on price, I think that's a step too far. MOL does NOT do complexity, one aircraft type, no sophisticated booking, cheap as chips, no connections. The FR culture would get f***ed up overnight the day they did that.
It depends....
Dunno - Book an Aer Lingus TA flight and get a voucher that entitles you to a Ryanair flight to Dub / SNN with checkable bag allowance / reserved seating and priority boarding and its cheaper than a direct as not a connecting flight.
No UK tax on flight as separate flights and fares a lot lower and think you may find quite a few takers.
Relatively easy to have 2 separate brands from same company.