This is my current irbrc file. Lots of helpful things I've collected over the years. Enjoy

.irbrc

script_console_running = ENV.include?('RAILS_ENV') && IRB.conf[:LOAD_MODULES] && IRB.conf[:LOAD_MODULES].include?('console_with_helpers')

if script_console_running # log SQL for Rails 2
  require 'logger'
  Object.const_set(:RAILS_DEFAULT_LOGGER, Logger.new(STDOUT))
end
# log SQL for Rails 3
ActiveRecord::Base.logger = Logger.new(STDOUT) if defined? Rails::Console

require 'bigdecimal'
if defined?(BigDecimal)
  BigDecimal.class_eval do
    def inspect
      to_s
    end
  end
end

# autoindent of code while typing it
IRB.conf[:AUTO_INDENT]=true

ANSI = {}
ANSI[:RESET] = "\033[0m"
ANSI[:BOLD] = "\033[1m"
ANSI[:UNDERLINE] = "\033[4m"
ANSI[:LGRAY] = "\033[0;37m"
ANSI[:GRAY] = "\033[1;30m"
ANSI[:BLACK] = "\033[0;30m"
ANSI[:RED] = "\033[31m"
ANSI[:GREEN] = "\033[32m"
ANSI[:YELLOW] = "\033[33m"
ANSI[:BLUE] = "\033[34m"
ANSI[:MAGENTA] = "\033[35m"
ANSI[:CYAN] = "\033[36m"
ANSI[:WHITE] = "\033[37m"

# Build a simple colorful IRB prompt
if IRB.conf[:PROMPT]
  IRB.conf[:PROMPT][:SIMPLE_COLOR] = {
  :PROMPT_I => ">> ",
  :PROMPT_N => ">> ",
  :PROMPT_C => "?> ",
  :PROMPT_S => "?'> ",
  :RETURN => "=> %s\n",
  :AUTO_INDENT => true }
  IRB.conf[:PROMPT_MODE] = :SIMPLE_COLOR
end

# Some default enhancements/settings for IRB, based on
# http://wiki.rubygarden.org/Ruby/page/show/Irb/TipsAndTricks

unless defined? ETC_IRBRC_LOADED
  # Setup permanent history.
  HISTFILE = "~/.irb_history"
  MAXHISTSIZE = 100
  begin
    histfile = File::expand_path(HISTFILE)
    if File::exists?(histfile)
      lines = IO::readlines(histfile).collect { |line| line.chomp }
      puts "Read #{lines.nitems} saved history commands from '#{histfile}'." if $VERBOSE
      Readline::HISTORY.push(*lines)
    else
      puts "History file '#{histfile}' was empty or non-existant." if $VERBOSE
    end
    Kernel::at_exit do
      lines = Readline::HISTORY.to_a.reverse.uniq.reverse
      lines = lines[-MAXHISTSIZE, MAXHISTSIZE] if lines.length > MAXHISTSIZE
      puts "Saving #{lines.length} history lines to '#{histfile}'." if $VERBOSE
      File::open(histfile, File::WRONLY|File::CREAT|File::TRUNC) { |io| io.puts lines.join("\n") }
    end
  rescue => e
    puts "Error when configuring permanent history: #{e}" if $VERBOSE
  end

  ETC_IRBRC_LOADED=true
end

class Object
  # Easily print methods local to an object's class
  def lm
    (methods - Object.instance_methods).sort
  end
  # look up source location of a method
  def sl(method_name)
    self.method(method_name).source_location rescue "#{method_name} not found"
  end
  def mate(method_name)
    file, line = self.sl(method_name)
    if file && line
      `mate '#{file}' -l #{line}`
    else
      "'#{method_name}' not found :(    Try #{self.name}.lm to see available methods"
    end
  end
end
 
# copy a string to the clipboard
def cp(string)
  `echo "#{string}" | pbcopy`
  string
end

ANSI_BOLD       = "\033[1m"
ANSI_RESET      = "\033[0m"
ANSI_LGRAY    = "\033[0;37m"
ANSI_GRAY     = "\033[1;30m"

def pm(obj, *options) # Print methods
  methods = obj.methods
  methods -= Object.methods unless options.include? :more
  filter = options.select {|opt| opt.kind_of? Regexp}.first
  methods = methods.select {|name| name =~ filter} if filter

  data = methods.sort.collect do |name|
    method = obj.method(name)
    if method.arity == 0
      args = "()"
    elsif method.arity > 0
      n = method.arity
      args = "(#{(1..n).collect {|i| "a#{i}"}.join(", ")})"
    elsif method.arity < 0
      n = -method.arity
      args = "(#{(1..n).collect {|i| "a#{i}"}.join(", ")}, ...)"
    end
    klass = $1 if method.inspect =~ /Method: (.*?)#/
    [name, args, klass]
  end
  max_name = data.collect {|item| item[0].size}.max
  max_args = data.collect {|item| item[1].size}.max
  data.each do |item| 
    print " #{ANSI_BOLD}#{item[0].rjust(max_name)}#{ANSI_RESET}"
    print "#{ANSI_GRAY}#{item[1].ljust(max_args)}#{ANSI_RESET}"
    print "   #{ANSI_GRAY}#{item[2]}#{ANSI_RESET}\n"
  end
  data.size
end

def migrate
  if defined? Rails::Console # turn off info logging for Rails 3
    old_log_level = ActiveRecord::Base.logger.try(:sev_threshold)
    ActiveRecord::Base.logger.sev_threshold = Logger::WARN
  end
  reload! && migations_ran = true if ActiveRecord::Migrator.migrate(Rails.root.join("db/migrate")).any?
  ActiveRecord::Base.logger.sev_threshold = old_log_level if defined? old_log_level
  migations_ran ||= nil # useful exit status
end