Let's start with the basics. RIDE is an editor for Robot Script files. Robot Script files are executed by the Robot Framework robot application. It is important to understand the difference between these two applications. One creates files and the other uses them.
Assuming for a moment here that you have some custom functionality in a Jar file that you would like to make available inside the Robot Script files test cases. If this is indeed the case then your focus should be on create Robot Test Libraries. In the documentation there are numerous java examples given for basing your own library on.
This means that you do not need Maven, unless you want to, to create your library. Also Eclipse IDE is not needed nor is RIDE. The library you create can be used by Robot Framework even when it's run from the command line. Feel free to use them for what they are good at: writing Robot Scripts efficiently and running Robot through menu options instead of command line windows.
When you are familiar with Eclipse and comfortable with it's features, then I can also recommend using the Eclipse Application RED which is a mature Eclipse add on for creating, running and debugging Robot scripts.
There are two ways of implementing a Java routine in Robot Framework and it depends a bit on how you run Robot Framework: Python or Jython. Running Python means that you need to run the Java in a separate JVM container and connect to it. The simplest way is via Command Line interaction. This may cause some security concerns. The second approach is to use the Remote Library approach. A good example of this is the Robot Framework Sikuli Library. An added bonus is that you can run Robot and your Java application on two separate machines/servers.
The Jython way has been discussed on StackOverflow before and I gladly refer to it. Please bear in mind that although the examples refer to RIDE, the Library org.company.Keywordlibrary
works in all IDE's, provided robot framework is run using Jython.
In the Jython documentation it clearly explains how to setup an Eclipse based IDE with Jython. Do keep in mind that it's recommended to use the same interpreter in development and production. I.e. if you create your scripts in Jython, do run them in 'production' also using Jython. Otherwise you open yourself up to interpreter differences where functionality does not work or behaves differently then expected.
An alternative approach with Jython is the all-in-one Robot Framework Maven JAR plugin. Adding a custom Java Library in this setup is not too difficult. A nice short tutorial can be found on the blog of Valerio Bruno. If this is your preferred route for development, then you may want to read the RED howto on integrating the Robot Framework Maven jar.
Working on your custom functionality is likely going to be a independent project in Eclipse, but can be linked to an Robot project via the Class Path settings in red.xml if you use RED or the CLASS_PATH environment variable.