As with most flavors of Ruby, the easiest, cleanest and least painful way to install jRuby is with RVM, the Ruby Version Manager. And although jRuby is different than MRI, the official Ruby interpreter, it can be installed just as easily, arguably easier.
jRuby has only one real prerequisite and that's Java. But thankfully, OS X comes with Java preinstalled so there's no work to be done there. On other platforms, you may have to install Java first.