The last few months JRuby have used something called JNA to introduce some functionality we couldn't do before. This mainly includes POSIX file operations of different kinds. I've just spent a few days getting a basis for UNIX Domain Sockets working in JRuby, using JNA, and I've gotta say that I'm incredibly impressed by it. For simpler function calls, it just works. For more complicated stuff you might have to set up structures and so on, but it's still incredibly slick. The best part: no native compilation needed.
Of course, this has some performance considerations, but it's still better than nothing.