In Opera 7.5, go to Tools > Preferences... > Programs and paths and click the "Add" button under "Protocols". This will open a dialog box called "Protocols". Now, simply enter the protocol in the "Protocol" field without a colon or slashes (for example, you can enter "irc", but not "irc:" or "irc://"), and either set Opera to use the system default application to handle this protocol, or specify one in the "Open with other application" field.
