On gentoo installing irssi is quite simple:

root$ emerge irssi

Here's a little list:

/connect irc.freenode.net               # Connect to a server
/join #gentoo                           # Join the gentoo channel
/join ##linux                           # Join the linux channel
/nick foobar                            # Change your nick to "foobar"
/query foobar                           # Opens a private message, you type the msg
/wc                                     # close a channel or message window
/part                                   # Nicer way to leave a channel
/disconnect                             # Disconnect from an IRC server
/exit                                   # Quit IRSSI itself
/ignore foobar                          # Ignore someone
/ignore foobar MSG                      # Ignore private messages from someone
/ignore ##gentoo ALL -PUBLIC -ACTIONS   # Ignore parts, joins, quits
/rmreconns                              # Stop trying to reconnect to a server :/ 
/topic [space][tab]                     # This autocomplets the topic
/topic                                  # This just prints out the topic
/SET autolog oN                         # Turn on autologging
/save                                   # Save settings
/q foobarnick                           # Start a private message
	

Here's a really nice IRSSI resource. along with a little excerpt:

"If you accidentally freeze screen (usually this happens if you hit ctrl+s or ctrl+a then s) you can fix it by hitting ctrl+q or ctrl+a then q. Another way to potentially break screen is if you it the pause/break button while using putty or ctrl+z, then you simply need to type fg at the command line to bring your irssi session back. The fg trick also works should you manage to make anything else in screen dissappear with ctrl+z or pause/break."