According to the Air Transport World EI will be taken the A350-900 Regional. They will take all 9 and there whole long haul fleet will be A350. If the regional wasn't offered they wouldn't of taken 9 of the regular 900. Main differance is they are lighter weight and better suited to short-medium range routes.
Aer Lingus to take nine A350-900 regionals | Airframes content from ATWOnline