or for a more SLIMEy experience, you can try ScreenShell, which allows you to send code from Vim in one GNU Screen window to another running irb, or the interpreter of your choice.
https://qht.co/item?id=1098876
If you just want to stick to improving the irb experience, the Pickaxe book has a good section on customizing irb:
* wirble: for syntax highlightning
* bond: for more advanced autocompletion
* utilitybelt: for pastebin-like commands
* sketches: for editing and evaluating code easily
* boson: command framework for creating libraries of commands to be loaded as needed
* hirb: framework for associating views per class