As per the terms of
Etihad Guest Platinum status, access to the First Lounge at AUH requires you to be travelling on Etihad metal.
Air Berlin passengers only get access to the Business Class lounge at AUH when they are travelling in business class or hold topbonus Platinum status. All other AB customers are sent to the Al Reem lounge.
I would imagine, though, that EY Guest Platinum will get you into the Premium (NOT the First) Lounge as it corresponds to topbonus Platinum.