IRC is a chatting protocol. I am assuming that you mean mIRC, the chat programme...
Are you running any scripts in mIRC? Especially scripts that are malicious. It's possible to get mIRC to issue a shutdown for Windows. You have hardware similar to mine, as I run an AMD system and I can say that the setup is quite stable.
mIRC V6.03 has a slight memory leak when running certain types of scripts too, such as when running hash tables. This shouldn't cause your machine to restart though as you have a tonne of RAM. One question... Is your machine running NTFS partitions? If not I do recommend it unless you have a specific reason for not wanting them. Once again though, this isn't critical in this instance.