Just to add to David J's points above, Aer Lingus is hugely unattractive to any purchaser. 55% of its shares are held by persons that a purchaser would not want on its own share register, namely the Irish State and Ryanair. The Irish State would dearly love to sell its stake, as it's so desperate for cash right now (all State and semi-State bodies are potentially up for sale). However, which airline would want, or could afford, to make an offer at least 55% in cash (I'm assuming all other shareholders would accept a 100% share swap - very unlikely actually)? Even if there is an airline that would do this, which one could obtain the necessary funding?
Ryanair has already written down its Aer Lingus holding to current market value. As and when the Irish economy improves, and air travel does so in tandem, Ryanair will write back its losses. Ryanair shareholders are pretty sanguine about the losses, I suspect. If nothing else, Ryanair protected itself against competition in one of its key markets (remember when it first bought the shares Ireland was booming and a "proper" owner of Aer Lingus could have provided a challenge to Ryanair). Keeping hold of 30% gives Ryanair a de facto veto on where the Irish State's holding eventually ends up.
I still think Aer Lingus' entire future looks dubious. They've virtually no presence outside Ireland, Ryanair's presence in all its facets makes attracting strategic partners very difficult, and the 25% State ownership causes its own challenges, both in terms of ownership and EU competition/aid rules. Very hard to see how Aer Lingus can remain as anything other than a shrunken reminder of a bygone era in airline management. If Aer Lingus disappeared, would anyone actually notice?