There's another cool gem called awesome_print by michaeldv (Article).

I tried it out and I couldn't figure out how to customize it without editing the gem itself. So I forked the gem, which can be found on my github account.

To customize the configuration, add the following lines in your .irbrc

require 'ap'

AwesomePrint::OPTIONS.merge!(
  :indent => 2,
  :multiline => true
)