Book RyanAir as soon as you know your dates and you think it is a good price. D20 has been a huge RyanAir user during her year abroad, her lowest RT flight has been $28 and her highest ~$100. Those were with only taking a personal item that could fit in the underseat space.
If you are planning on taking real luggage with you - definitely pre-purchase luggage room or check to see which type of ticket is best for you. If you do have a lot of luggage - RyanAir may not be all that economical to fly and you may want to look at other carriers.