I doubt Aer Arann is a target for Aer Lingus, as it would offer no benefit in terms of expansion given that Aer Arann already operate solely under the Aer Lingus Regional brand. They also have a lower cost base than Aer Lingus, and Christoph Mueller himself has pointed out that when a larger airline acquires a smaller airline, the smaller airline will inevitably assume the higher cost base of the larger airline, which in Aer Arann's case could effect the viability of the Aer Lingus Regional operation.
If, as Christoph Mueller says, Aer Lingus wants to "consider small asset purchases in order to help it expand", it would have to look to the continent.