Wolfden has very good points about IRC. IRC is a very valuable collaboration tool in many Open Source projects. But you need to understand and respect it's shortcomings.
IRC provides an opportunity for immediate communication among a globally dispersed group of people. It is often less cumbersome than M/L for simple issues, or issues that require a considerable amount of back-and-forth discussion. But IRC communication is very ephemeral and short-lived. If you over-rely on IRC for communication, important messages get lost in the noise or because people just aren't online. And IRC burnout is a very common problem among many open source projects. It just becomes too demanding to have to monitor IRC all the time. Many Open Source projects make much more use of their Mailing Lists for collaboration. Mailing Lists are much better when your group is spread out across many timezones. Mailing Lists allow for project contributers to have much more flexibility in how they allocate time to the project. It's also important not to forget about all the other tools that can allow people to participate. We have Bugzilla, Forums, Wikis, Blogs, Announcements, and even Facebook and Google+ (and I'm probably forgetting a few others). People should feel free to log into IRC at their discretion without feeling a need to be tethered to it 24 hrs/day, and the project should still have productive avenues for these people to participate.
