Hi,
providing authentication for another system sounds like a job for LTI. As long as the other system supports LTI too, this might solve your requirements.
The best way to start developing plugins for ILIAS IMHO is learning by example. Filter for repository plugins in theplugin collectionand study the sources. Have a look at the opencast-Plugin, as it includes AFAIK authentication for an external system based on course-membership.
If you are looking for particular functions within ILIAS to get IDs or roles, you can dive into the classes at ./Services (e.g. /Services/User/classes/class.ilObjUser.php). If you can't find what you need, you can also deliberately create your own SQL-Statements and get the information directly from the database. You can do in your own plugins whatever PHP has to offer. You are not restricted.
Best regards,
Christoph
