Where it is physically connected may not be a factor. If there is no check valve between the tank and your system then it is the same as if it were connected after the tank. If there is a check valve then the pump should not turn on until something in the house depletes the pressure so the pump can start. If you have the system wired somehow so that the pump can be operated when the irrigation system starts, then the question would be whether the pump can maintain pressure or if the pump can "overwhelm" the system and build up excessive pressure. Excessive pressure would be detrimental. Not having adequate capacity to maintain the irrigation system's pressure would cause a gradual loss in efficiency.