I think that
http://maven.jenkins-ci.org is just redirecting to
http://repo.jenkins-ci.org in order to be backward compatible.
The former URL was valid when we used Nexus as repository manager. Quite some time ago we switched to Artifactory so now the latter URL should be used only.
Note that the prefix public probably is a group that merges several repositories into one URL.
I'm using the following snippet to access the artifacts:
<mirrors>
<mirror>
<mirrorOf>*</mirrorOf>
<name>public</name>
<id>public</id>
</mirror>
</mirrors>
Ulli