I don't think there is any particular reason to learn any Java before learning Ruby.
I taught an introduction to programming course a year or so ago, and used Ruby and JavaScript as the languages. The students picked both of them up fine without any prior experience.
Why do you think you might need to learn some Java first?
Ruby is a stand-alone scripting language available on many platforms. Ruby is the implementation language of the widely-hyped Rails framework.
JRuby is an implementation of the Ruby language which runs on the Java Virtual Machine and integrates closely with Java.
Ruby and JRuby are deliberately very similar, but there are some subtle differences due to the different underlying implementations and the process of change and catch-up in language specifications.