You said ‘I’ll make changes to be programmed manually without going online’, which made me curious why you still need ENC28J60 (the Ethernet controller) — I thought you mean program the controller with buttons and not with a web interface.
In any case, OpenSprinkler is designed to work at 3.3V. For example, the microcontroller, 74HC595, ENC28J60, microSD card, all work at 3.3V. It does have a 5V line because DS1307 and 1602 LCD require 5V, but these can both interface with a 3.3V microcontroller.
Ray thank you very much for the response, the Ethernet module will separately as optional, the irrigation controller will not use an embedded web server, I will use TCP protocol from a webserver to see the status of irrigation and command it remotely (Similar to the operation of Open Sen.Se). I find it much more efficient due to the limited memory capacity of the microcontroller.
But as I said above, is optional and external to the main board.