There are two main ways garden irrigation systems get the water pressure: The first is a watering system that runs direct off you mains water supply and the second is a pumped system.
The difference in systems (apart from one has a tank and pump) is that the systems that run off the mains are limited by the flow at a given pressure the local water supplies gives which is normally poor and subsequently minimises the size of the system. Also you can do a dynamic flow test on one day and the water board can change the pressure the next. In addition, normally to comply to Water Regulations pop-up sprinklers cannot be connected direct to the mains. Please consult your local water company.