"You are selecting to book a flight for travel today. Only flights due to depart more than 6 hours from the current time are available for booking online. For same day travel you must check-in online and print your boarding pass after booking completion.
Online check-in closes strictly 4 hours prior to scheduled departure."
Also, Irish rail say the platforms close 2 minutes before departure, Ryanair say "You should be at the boarding gate at least 30 minutes prior to scheduled departure. Boarding closes 20 minutes prior to departure." Make that a minimum of 40 minutes if checking in luggage.
Irish Rail deliver you to train stations, Ryanair deliver you to airports. It will depend on your ultimate destination which is better for you.
Other Ryanair fees
http://www.ryanair.com/en/questions/table-of-fees
Terms and conditions
http://www.ryanair.com/en/terms-and-conditions
Ryanair, 3 flights per day on Saturday-Sunday, 5 flights per day on weekdays.*
http://www.bookryanair.com/skysales/FRTimeTable.aspx
Irish Rail, 10-11 trains per day on Sunday, 14 trains per day other days (+1-2 change at Mallow).
http://www.irishrail.ie/your_journey...timetables.asp