Please describe how you expect a wireless siren to be installed and or powered?? Run it off of batteries?? How loud for how long??
You might consider sending an X10 command but then if the power is off how will the signals get there??
You might consider a radio signal but then you will need a transmitter and a receiver. Receiver will need power.
Do you see a common word here?? "Power" is the word.
You will need power and power translates into needing to have wires.
Find a way to run some wire or re-think where you want that siren.