You have to understand the difference between the interface and the protocol. The interface is the IR receiver to the microcontroller and only carries binary data. The protocol is the construction of the binary data that lets it carry useful information, in this case the commands.
The schematic you show can use any protocol, it is up to the software to recognize what arrives at the microcontroller pin. The only way to discover the data format is to use an oscilloscope or even better, a logic analyzer. You should be able to identify the part of the data which is constant (the device address) and the part which is different for each key you press (the command). The position of the bits will help you discover the protocol being used.
Brian.