The costs associated with setting up another airline within the EI group could well sink Aer Lingus. The capital requirements alone in terms of getting a new AOC, sourcing equipment etc. would make a bad cash situation even worse. The other problem with establishing another carrier is the simple fact that the European market is flooded with LCC's. We have Ryanair, EasyJet, Air Berlin etc. and these carriers have such a foot hold in the market that it's near impossible to establish a new carrier on a sound financial footing.
As Aer Lingus LGW effectively operated as a LCC with T&C's more akin to EasyJet, it goes to show that establishing a new carrier is just throwing good money after bad.
Finally, the management time that would be devoted to such a venture would leave a big gap in the management of the Aer Lingus group.