I renamed derby.jar to derby-10.0.2.1.jar and derbynet.jar to derbynet-10.0.2.1.jar and copied both files to .maven/derby/jars. However, this did not fix the problem. The build appeared to be looking for the jars elsewhere.
Then looking in the modules/derby/project.properties file, I found the repo to be:
maven.repo.remote=http://cvs.apache.org/repository
In the almost hidden location http://cvs.apache.org/repository/incubator-derby/jars directory there is a derby-10.0.2.1.jar file and a . derbynet-10.0.2.1.jar file. I say "almost hidden" because I didn't find any "derby" directory as in the other repos, but after a bunch of messing around and looking in project.property files and project.xml files, I eventually found that I needed to look in "incubator-derby".
I also discovered that I needed to copy the above jar files to .maven/repository/incubator-derby.
Subsequently, the Derby module built OK.
The original error was:
+----------------------------------------
| Executing default Geronimo :: Derby
| Memory: 3M/6M
+----------------------------------------
Attempting to download geronimo-kernel-1.0-SNAPSHOT.jar.
Artifact /geronimo/jars/geronimo-kernel-1.0-SNAPSHOT.jar doesn't exists in remote repository, but it exists locally
Attempting to download geronimo-system-1.0-SNAPSHOT.jar.
Artifact /geronimo/jars/geronimo-system-1.0-SNAPSHOT.jar doesn't exists in remote repository, but it exists locally
Attempting to download derby-10.0.2.1.jar.
WARNING: Failed to download derby-10.0.2.1.jar.
Attempting to download derbynet-10.0.2.1.jar.
WARNING: Failed to download derbynet-10.0.2.1.jar.
