irbrc.rb

# IRB Classic prompt
IRB.conf[:PROMPT_MODE][:CLASSIC] = {
  :PROMPT_I => "%N(%m):%03n:%i> ",   # irb(main):001:0>
  :PROMPT_S => "%N(%m):%03n:%i%l ",  # irb(main):003:0"
  :PROMPT_C => "%N(%m):%03n:%i* ",   # irb(main):005:0*
  :RETURN => "%s\n" # used to printf # 
}

vs 
# IRB Simple prompt
IRB.conf[:PROMPT_MODE][:CLASSIC] = {
  :PROMPT_I => ">> ",  # >>
  :PROMPT_C => "?> ",  # ?>
  :RETURN: => "%s"
}