In the beginning, there was one
IRC network. As more servers, users, abusers, and disagreeing operators were added, it turned out that the IRC client/multiserver architecture doesn't scale to arbitrary size. So new networks were formed:
DALnet and
Undernet. The main network remained intact - increasingly unstable - until 1993, when a group of mainly USA-based admins split off to form
EFnet; the mainly European remainder was named
IRCnet.
These original networks are still around, and a number of other general-purpose networks have gained popularity as well.
Before commercial spamming existed you would continually be spanned with advertisements for new IRC networks: "Come and visit Lusernet, it's so fast and so stable! We are the operators, and we have 4 servers already!' Right, no wonder it's fast when all you can see is 4 users and their dog. The point of IRC is being able to talk to the whole world!
On the large networks, the issues with stability (mainly due to abuse and sabotage) continue, and small, community-based networks such as slashnet are much more significant now.