About a year ago, I was introduced to the joys of using irssi and screen to access irc servers. Before that time, I'd usually used graphical clients like Colloquy, and I always enjoyed getting Growl notifications when someone mentioned a word or string that I set up as a trigger.
Once I started using irssi in screen, I found that the visual bell in screen didn't get my attention quickly. Luckily, someone in the #slicehost channel let me know about screen's audible bell. You can flip between the visual and audible bell with CTRL-A and then CTRL-G. If you keep repeating that key combination, you'll switch back and forth between the two (with a status update at the bottom left).
You can also set up your visual bell configuration in your .screenrc via some configuration parameters:
vbell [on|off] vbell_msg [message] vbellwait sec
