You got it !!! A new brand aimed towards your new market works a whole lot better than a brand people link with flights to Ireland
.
If you asked the people on the streets of London and areas around if they know where AerLingus flys most would say WHO and the the rest would say Ireland.
Thats not forgeting the people in European Cities who wish to fly to London they probabley cant even say Aer lingus or spell it, thats nothing against them its just an un-friendly European name.
By rebranding the news spreads fast of a new player in the market even if its not new people will see a new Airline.