Add this to your ~/.irbrc
file (creating one if it doesn't already exist):
require 'irb/ext/save-history'
IRB.conf[:SAVE_HISTORY] = 200
IRB.conf[:HISTORY_FILE] = "#{ENV['HOME']}/.irb-history"
Boom! Last 200 commands are saved, such that you can up arrow / command-p or down arrow / command-n to move through your ruby console history!
Whoo!