Arduino-irrigation is a GSM-based remote controller for irrigation systems.
To build and install it, I use Ino. You may be able to do it from the Arduino IDE, but I haven't tried it.
To build and install using Ino, copy the GSMSHIELD directory to lib/, tweak the pin configurations to suit your pins (the defaults should be good, though), and run:
ino build ino upload
That will build and upload the software to the Arduino, and in a minute or so you will be able to call the Arduino and have it toggle the relay on or off (as well as send you SMS when the inputs change).
Arduino-irrigation uses authorization, which means that you will have to store up to two numbers on the SIM card (in positions 1 and 2). Those two numbers are the only ones that will be able to control it. Whenever an input event happens, Arduino-irrigation will send an SMS to the number in position 1, so that will be your main number.
The main functionality of Arduino-irrigation is that you can call it and have it toggle your system (a relay) on and off. It supports push-button relay systems (two relays, each is activated for 0.5 seconds, one turns the system on and one turns it off), for a system that has two on/off buttons, and single-relay systems that just keep one relay on while the system is on, and turn it off to shut the system down.
It also supports an "enable" input that enables SMS sending (you turn it off if you're changing things manually so you don't get notified all the time), an "in operation" input that tells the software whether the system is on or off, and three "error" relays that send SMS if they are triggered (for various failure conditions).
For a better description of how it all works, the code is (hopefully) easy to understand and concise, so I will refer you to that.