how to control the pump's pressure for an irrigation compter system?

Answer 1

Assuming you are using a centrifugal pump:

The simplest way would be to have a regulating valve or to just control the RPM of the pump. Different pumps will produce different pressure differentials according to the input RPM. By acquiring a pump chart from a manufacturer, you can match RPM and pressure differential values. The number of RPM you produce is determined by the motor you're using.

Answer 2

There are 3 basic ways to control a system's pressure:
1.) install a control valve to restrict the outflow from the pump. (this method can lead to pump damage if the flow is below the minimum required for the pump for a long period of time)
2.) Install a bypass line controled by a control valve to bleed off pressure and return it to either the source your pumping from or the pump suction.
3.) Install a variable frequency drive set to modulate with pressure (note the pump will have a minimum speed that it will need to turn to function properly, so choose the pump wisely and make sure you use an inverter duty motor to handle the extra load from running on a VFD)