Actually...
What would probably work better and be much more user friendly would be multiple connect dialogs.
Go to options and connect and find a list of connections with, add, edit and delete boxes.
When you add or edit a connection, you get something similar to to the connect dial with full name, email address, nickname, alternative, invisable.
What would also make things simpler would be to have all the server settings on that same page: Server description, address, ports, network name, password and a button to let you browse the server list to choose one instead of just typing the info.
And *then* a check box for connect on startup.

And then also a button to open the perform box for that connection.
You might use a different nick and password on different servers, and you might need to perform different commands when you connect to different servers.
I use mIRC a lot to help people get people connect to my IRC channel and it would be a lot easier for me if I could guide them to one dialog box to enter all the connection settings rather than going to the connect dialog for user info, the server browser to add and select a network and server and the channels folder to add the channel, select the network it's on and make it join on[/b] connect.
As per letting you still use "irc:" urls correcty, you could have a default first connection where you take the user info from for those links.
Seems like a big project to impliment, but after adding multiple server capability, it's probably not a bad idea to go over the interface with multiple servers in mind from the beginning rather than adding the functionality on top of a single server interface.