$ echo "IRB.conf[:PROMPT_MODE] = :DEFAULT" > ~/.irbrc
$ irb
irb(main):001:0> quit
$ echo "IRB.conf[:PROMPT_MODE] = :SIMPLE" > ~/.irbrc
$ irb
>> quit
http://ruby-doc.org/stdlib-2.0/libdoc/irb/rdoc/IRB.html
http://ruby-doc.com/docs/ProgrammingRuby/
http://stackoverflow.com/questions/6039292/how-to-format-irb-command-prompt