The problem isn’t the channel parameter in the method with the same name as a parameter of the class, it’s still using the one of the method unless you call it up using the this prefix. It’s still a good idea to get a better naming convention. ReSharper will warn for issues like this on the fly.
I think something goes wrong in the actual code calling the method, or even better at the constructor.
Can you show use the line of code where you are constructing the IrcClient?