The Logo! by Siemens is a pretty simple controller which can be used for irrigation control.
The version I have has 4 digital inputs (e.g. for a fill level switch), 4 analog inputs (e.g. for SMT100 soil moisture sensor) and 4 relay outputs (e.g. magnetic valves). For small installations this may be sufficient and a pretty low cost solution. I am still struggling with the web based control. The Logo! has less options than a full-sized plc, but price performance ratio is quite well.