Originally Posted by ISTFlyer

Yes, you would be in the international terminal when you arrive but not in the departures hall. You would find yourself in the transfer level and to access the departures level where the lounge is located, you would need a valid international onward boarding pass as to reach the international lounge from where you arrive there is a security check where boarding passes are checked by security.

Plus, TK doesn't offer lounge access based on the class of the arriving flight. If you are flying domestic and if you would like to use the Domestic Lounge, you either need to hold TK M&S Classic Plus & higher status, hold *G status or fly that leg in Business Class.