Buy your flights on Ryanair's website. Although it is possible to buy tickets at the airport, this is more expensive. Ryanair doesn't sell through travel agents and always has deals advertised on its home page.
Look for flights that depart between noon on Monday and noon on Thursday, or after noon on a Saturday. The cheapest tickets are for these times. Tickets for very early-morning departures are often the cheapest; these may involve spending the night in the airport, if the flight leaves before public transport starts running.
Buy your flights well in advance. The cheapest fares on Ryanair must usually be purchased at least 14 days before travel; this is sometimes extended to 28 days.
Keep your plans as flexible as possible. Choose your destination and use the "Flexible Search" selection on the website to see when the cheapest flights are, then arrange your trip around them.
Check the website regularly for special sales. The lowest fares are often promotional offers available briefly on the Ryanair homepage.