Relay before or after power transformer
It's not possible to predict what will happen if you break the circuit between the converter and the lamp. The converter may fail catastrophically if it is switched on without the load it was designed for. 

If you could be confident that the converter is nothing more than a step-down transformer, perhaps with a load limiting resistor, then you could insert the relay between the converter and the lamp. But since this product is touted as a superior energy saving device, it is likely to use a TCBH1 power technique that requires the lamp to be permanently connected. 

Therefore I recommend inserting the relay between the power socket and the converter. 



1 Too Clever By Half