For autocompletion add

    require 'irb/completion'

to your ~/.irbrc. If you want auto-indentation:

    IRB.conf[:AUTO_INDENT] = true

If you're a Vim user, you can edit code inside of a Vim buffer with the interactive_editor gem:

    http://vimcasts.org/episodes/running-vim-within-irb/

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.

    http://news.ycombinator.com/item?id=1098876

If you just want to stick to improving the irb experience, the Pickaxe book has a good section on customizing irb:

    http://ruby-doc.org/docs/ProgrammingRuby/html/irb.html