Mains water supply 
Rain filled 
Water tank 
Mains driven Electronic Timer
battery operated tap/timer switch 
long distribution hose 
sprinklers, soaker tubes, and drippers 
12v car Battery charger
12v Pump 
long distribution hose 
Obviously the mains water system is the easiest to build, and various companies make sprinkler systems. Hoselock being the biggest and DIY stores like B&Q sell them. If your water is metered then this could be expensive especially if it leaks, but it is simple. Must be turned off during hosepipe bans, which is generally when you need it most.
  Ecologically more sound, I think it could take bath water or washing machine water depending on the kinds of soap you use, but I've never tried that. 
    It's best to have it operate at night, people in the garden are less likely to get wet and the water does not evaporate as quickly, so it soaks in better. You may have to adjust to your cat's nocturnal habits.
   There are float switches that will disable the pump if the water gets low, but they generally need a 12volt relay as they cannot handle the current that the pump needs.
   The tank will be siphoned dry if you don't have a sprinkler nozzle above the tank water line. 
   You will need a submerged filter of some kind, or the nozzles will block up.
sprinklers, soaker tubes, and drippers 
Jim's Garden or greenhouse water sprinkling systems
1. Simple Brute Force method, solving it with money
2. Rain water system, more ethical, but more complicated and so less reliable.