To connect to a server called irc.testme.com with the nick called iamatester you can give the following command at your shell prompt:

irc iamatester irc.testme.com

You can also export variables to your profile temporarily for your session, so you only need to type irc next time on, and you will connect to the specified server with specified nick. The following command will export the nick as iamatester and the server irc.testme.com to your profile:

export IRCNICK=iamatester IRCSERVER=irc.testme.com

After you give the above command, all you have to do is type "irc" (without quotes) and you will be connected as iamatester to the server irc.testme.com

To make the changes permanent, you can edit your .profile file (usually ~/.bash_profile or ~/.zprofile) and put the following line in it permanently and save it:

IRCNICK=iamatester IRCSERVER=irc.testme.com

Similarly, just like the example above, you can also specify the "IRCNAME" and "IRCUSER" variables for your IRC sessions. The "IRCNAME" is the name that appears in your /whois output and the "IRCUSER" is what appears before the @ sign in your /whois