Generally these systems are more suited to new builds than existing houses due to the installation of a large tank in the ground and plumbing.
Grey water systems are where the rainwater that falls on your roof is collected in a large tank and used in your toilets, washing machine and garden tap. The tank is usally buried in your garden.
A big factor in deciding if you want to install one of these systems is whether you will collect enough water to meet your needs. This will depend on the average rainfall in your area (e.g. Galway gets twice as much rain as Dublin) and the area of your roof.