IRC is just a place where people gather to chat. On some games that lack games but has enough players online [like xonotic], people can organise games via IRC channels. There' also public skype/ventrilo/teamspeaker etc. servers for some cases such as AussieAssault would have its community chatting on its eclan channel.
Overall, IRC is just a place to chat. There's nothing much to it. It would be sweet if we could also have a built-in client like how unvanquished/"tremz" does.