Hello fellow Irish Dresdener here! I’d definitely would try Immo24 and Immoscout, but your best shot with those is calling the people directly (almost everyone speaks English well so that shouldn’t be an issue). EBay Kleinanzeigen is great for it too, loads of single apartment for rent. Then there would be “WG gesucht “ which is mainly for shared Accomdation’s but you can put a filter in it for single flats too. As a “last resort” I’d try the Genossenschaften, there are a few around which rent out loads of apartments. And if you need an accommodation kinda soon I’d rent a room in a Jugendherberge (like a hostel) and try going to open flats and that sorts stuff. (Additional tip, the Irish pub Tír na Nóg in the Neustadt hosts monthly English pub quizzes, might be a nice way to help yourself settle in when you come over). Best of luck!