If you are using a low voltage (12v) lighting system in the garden and it has a transformer which plugs into the mains, you can do what I have done (assuming you want the lights to come on at night only).. use a timer switch that plugs into the socket, then plug the transformer into that. You may need to adjust the timer (weekly) to compensate for daylight hours.
If you are referring to a low voltage lighting system indoors (eg. kitchen) does the power Does come from the mains or from the lighting circuit?