IMHO, the Ryanair stake will be just one more reason, amongst a number of significant other reasons, why Aer Lingus is not that attractive as a buy for another airline, but it would not be a serious impediment to making it 'a buy' to professional investors.
The big problem would be a sale to any firm that would be inclined to asset strip or look for very quick returns on its investment.
Despite the structure of share holding, excluding the State's stake, and the long tradition of a somewhat fractious nature to management/union relations, EI has a decent chance of a being a long term, profitable operation, all be it as a relative minnow in the airline sector.
JAS