you can just do anything you like with it! its to IRC what mutt is to email, what centerIM is for IM. perl scripting, multiple windows, bouncing, colours and text style and so on. if you like to work in text-only mode this is your choice for IRC.
the settings you see in the pic are quiet easy!
from top to bottom
channel topic
statusbar with [current channel][nick][activity][dontknow maybe channels]
channel
textinput

the last line is not a typical irssi statusline, as far as I see its a screen statusbar (could be part of irssi but I doubt it)
means: irssi is actualy not running at the n900!
he ssh'ed to another machine were he uses "screen" (another powerfull console tool) to have multiple shells and in one of those is irssi running