Let's be safe
On a Mac, IRCing is a safe and enjoyable experience as there are no current exploitable ways of attacking a Macintosh remotely via IRC, or through the the network connection itself.
That being said, with the increasing popularity of the Mac it is sensible to take care and exercise common sense when using IRC, even on a Mac.
Don't accept unsolicited files
The most important rule is not to accept file transfers from people you don't know unless you have requested the file.
These unsolicited files are almost certainly attacks targeted against the Windows operating system, since the malicious users don't know you are using a Mac.
Files with Windows viruses are completely harmless against a Mac due to many different technical reasons, but you should still not accept unsolicited files.
Avoid clicking on web page links posted by strangers
There are many websites that are set up to infect machines with viruses and spyware. Again, these attacks are almost certainly targeted against Windows. However, a web server run by a malicious user can determine your chosen operating system and potentially serve a targeted virus if an exploitable way is found with which to attack a Mac. So, avoid clicking on a web page link if you have any reason to suspect the poster.
Be careful about giving out personal information
The people you talk to on IRC are strangers and you will only gradually get to know them through their messages. Even so, appearances can be deceiving and you are cautioned against giving out personal information without care.
The vast majority of IRC users are decent, honest people and many, many people have first met on IRC, and later in real life, outside of IRC to become real friends. Still, use caution and common sense. If someone seems too friendly, and too interested in personal details, just disengage or put the person on ignore.
Keep your operating system up to date
Apple frequently releases updates to the operating system to close potential security holes. As I mentioned earlier there are no current exploitable ways of attacking a Mac remotely but it is reasonable to expect that one will be found in the future.
When you are on IRC you are interacting with others and their computers. The security updates will prevent potential future remote attacks against your computer.
To check for updates to the operating system, go to the Apple menu in the upper left corner of the screen, and select "Software Update"