OS X has a bunch of IRC clients, none of them are really as big in popularity as mIRC is for windows, instead they're spread out among a few different clients.
The one of the more well known IRC clients for OS X is
Xchat Aqua. This is the same as the Windows and Linux version of XChat, only built for OS X Aqua interface. You can use the TCL, Python, Perl, and Ruby scripts.
Another IRC client for OS X is
Conversation. Conversation's fairly easy to set up - open accounts and enter information. It supports using AppleScript to set up various scripts - displaying iTunes song or system info for example.
A third IRC client for OS X is
Colloquy Colloquy also supports SILC Connections (
Secure Internet Live Conferencing protocolAgain it's fairly easy to set up and supports AppleScript for creating Scripts. It's got a fairly large amount of extra's to choose from, more theme's then xchat aqua or conversation, but less scripts then xchat aqua.
Snak Is another option. This client is the easiest to script as it comes with a large amount of presets and an option to automate stuff. It's also really easy to set everything up with Snak. Worth checking out if your just starting IRC.
There's a bunch of other irc clients for OS X - if you want, there's a decent list located
Here.