What causes low water pressure from a deep well to a sprinkler system but not to the house?

More than likely it is the size of the (supply) pipe from the pump to the system. If you have a larger supply going to the house and less demand (one facuet on vs. 4 sprinkler heads on) the presure will be greater in the house. If you increase the size of the supply to the system it will help. Just remember presure and volume work in concert with each other. GOOD LUCK!

ANS 2 - Systems like this always work better if the well pipe goes directly to the pressure tank. At the tank outlet you have two pipes, one to house, one to irrigation. A larger tank than normal is necessary too, at least a 100 gal tank, operating at 50-80 psi is desirable.