I just read the following paper which describes an interesting study that was conducted regarding IRC: http://www.enre.umd.edu/content/rmeyer-assessing.pdf
The researchers created several IRC bots with different names - some female, some male, and some ambiguous. They put the bots in several high traffic IRC channels, and had them record all the private messages they received. The bots themselves were completely silent.
The bots with male names received an average of 3.7 private messages per days that were sexually explicit or threatening. The bots with ambiguous names received an average of 24.9 such messages per day. The bots with female names received an average of 100 such messages per day!
This is a very sad statistic, and probably goes a long way towards explaining why there aren't that many women on IRC these days.
On a happier note, if you want to hang out on IRC and not get sexually harassed, you can always join #wikimedia-gendergap!
Ryan Kaldari