When designing a rainwater harvesting system for irrigation, key factors to consider include the catchment area, rainfall intensity, storage tank size, and irrigation demand. The catchment area should be large enough to collect sufficient rainwater, while the storage tank size should be adequate to store the collected water. Irrigation demand should also be considered to ensure the system can meet the water requirements of the green space. Additionally, factors like roof material, gutter and downspout design, and first flush devices should also be taken into account.