What you are looking at buying is PLC controller (and a pricey one at that) which could be a device on a home automation system, although you will probably discover most will not support it (but since it has its own UI you might not care). I've looked at using a PLC to replace my irrigation controllers as I have unusual requirements not met by conventional timers, but its hard to justify the cost. If all you want to do is control an irrigation system there are much better options that will integrate with home automation controllers. You do not need a PLC that supports RSA (Rivest, Shamir, and Adleman) 256 and execute the lesser SHA (Secure Hash Algorithm) 256 hash function both of which are overkill for anything you are doing. If you are thinking you might need to build something because you have to deal with pump switching and well recovery times and you are comfortable with programming you might want to look at the PLCs available from automationdirect.com.
Home automation controllers are mostly designed to handle lighting, small appliances, home entertainment, security, and the like. Lutron, Elk, and HAI all have decent controllers. Most can communicate with switches, thermostats, infrared devices, and devices that have Ethernet TCP/IP. Most support a subset of the following home automation protocols; X-10, Radio-RA, Z-Wave, UPB, and Insteon. They also do have modules for irrigation, but they generally expect a fairly vanilla setup. No one setup is right for everyone.
-rick