Use Roman numerals the same way you can use other numeric representations in Ruby

roman_numerals.rb

class Module
  ROMAN_NUMERALS = {
    'M'=> 1000,
    'CM'=> 900,
    'D'=> 500,
    'CD'=> 400,
    'C'=> 100,
    'XC'=> 90,
    'L'=> 50,
    'XL'=> 40,
    'X'=> 10,
    'IX'=> 9,
    'V'=> 5,
    'IV'=> 4,
    'I'=> 1
  }.freeze

  def const_missing(roman)
    return super unless roman =~ /\A[MDCLXVI]+\z/

    roman.to_s.scan(/C[MD]|X[CL]|I[XV]|[MDCLXVI]/).inject(0) do |sum, numeral|
      sum + ROMAN_NUMERALS.fetch(numeral)
    end
  end
end

roman_numerals_example.rb

MCMXCIX == 1_999
# => true

MCMXCX == 0x7d0
# => true

MCMXCIX + 2
# => 2001