Aer Lingus will stay as a seperate company just as BA, Iberia and Vueling are seperate within IAG. It is Irish in origin and Irish it will remain since that is what it plays upon with its IrishAmerican customers
The synergies will come backstage with aircraft and services procurement.
I am sure a lot of passengers from the regions, especially Scotland, will be most satisfied to fly Aer Lingus rather than BA and especially since they would enjoy US Border clearance in Dublin (or Shannon).
Look at the “where we fly” map covering the UK and you see so many destinations to/from Dublin, 18 if my counting is correct, quite a few of which do not have flights to LHR.
It is a win-win solution in my view.
“Ryanair looking to offload Aer Lingus stake”
(Travel Weekly 14-01-15)