What exactly is the benefit of this? Aer Arann is already operating on thin routes across the Irish Sea, and feeding Aer Lingus flights from the UK makes little to no sense at all with the exception of US flights and maybe the odd flight to Spain. Where it would make sense from a pax point of view is seamless connections to the regional Irish airports, but those routes are subsidized anyway and from what I understand the situation at DUB still makes this impossible.
So what comes into play other than having an additional distribution channel for existing flights?