Like all Arduino projects you need to go about this step by step.
First of all get the Arduino to transmit whenever a switch is pressed - that will be simpler than the IR decoder. That will allow you to develop the logic for controlling the transmission.
Then write a piece of code to blink an LED when the IR signal is detected. The blink will be equivalent to the switch press.
Then merge the two concepts.
I share @PaulS's suspicion that an interrupt is unnecessary. Interrupts can be very difficult to debug. Why not just check the status of the pin the LED input is connected to in every iteration of loop?
...R