It's been reported in the past that EI were examining the options for C-Series and E-Jets......could these find their way into the Aer Lingus Regional brand? The economics would be appealing for Aer Lingus given the lower cost base of Aer Arann and Aer Arann would presumably get access to routes and growth otherwise not achievable on its own.
Not unless Aer Arann were to buy the jets themselves, which won't happen.
Remember, all of the financial risk in the Regional franchise lies with Aer Arann. In turn, Aer Arann retain the bulk of the profits from the routes, with Aer Lingus being given a royalty fee for the use of their brand name, where they would've otherwise broke even or lost money operating the routes under their own metal, while also increasing the amount of transfer passengers onto their long-haul flights (where the real money is).
So assuming Aer Lingus make an order for regional jets in the future, they would become part of the 'mainline' fleet.