In either the cheddarbox or any available IRC prompt, type: /msg (cat's name here) (message). For example, if I wanted to send an urgent message to my cat, I'd type:

/msg Bokonon Listen, I'm sorry I left you behind when I moved, but you kept trying to suffocate my roommates in their sleep.

Keep in mind, however, that, like all normal human interactions with felines, anything not concerning food or being petted will most likely be ignored.