Its funny, I got seriously involved with PC's in the mid 80's, at that time Radio Shack had the Trash 80 (TRS80) which had accessories to do a multitude of household chores. You could turn on a coffee pot at a certain time, turn on/off lights, regulate your hvac/heat...etc all from the home computer or thru a dialup connection.
Anyway, the simplest way to control a sprinkler system is using a good old timer.
IF for some reason you MUST use a plc for this then leave the home computer connected to the plc and use a program like PCAnywhere, CLoseup etc to dial into the home computer and do
what is necessary.
This way there is no cost except for the PCAnywhere type software and a modem(which most peolpe have). This is assuming you have the plc software already loaded on home pc.