Hi Martin,
Your galaxy instance does not have to be accessible from outside. The GALAXY_URL is send to the external datasource to create a new url that your browser understands and that galaxy will use to retrieve the data prepared by the service. E.g. http://localhost/galaxy/tool_id=1?url=http://externaldata/dat1.txt. You only have to make sure that galaxy is able to reach the external service.

Cheers, Jelle