Colloquy can be pretty bad, but I've been satisfied using either Adium's IRC support (though I wish I could easily block the server message / nickserv spam) or LimeChat, which was the simplest/fastest standalone OSX IRC client at the time I was using a standalone client.
With Adium I've been pretty happy since I discovered the ability to turn off show/join messages on the history menu.
The other thing was the Renkoo message style (from preferences, messages) was much better than what I had before. Date and name to the sides, content in the middle.
I have a Mac mini logged in permanently so I can review logs and there I took the advice which was to right click and block chanserv and nickserv. Not quite brave enough to do that on my laptop.
