We suggest that you arrive at the airport at least 3 hours prior to flight departure for international travel, 3.5 hours if you have a rental car to return.
For domestic travel, arrive approximately 2 hours before your scheduled flight departure.
For travelers returning to the States from Dublin, please remember to allow time to clear US customs/immigration as this process is done in Ireland so that when you land in the States you are landing as if you had flown domestically.
For your peace of mind and to avoid problems you should monitor conditions such as traffic, weather or anything that may affect your check-in (such as traveling during a holiday).
If you miss a flight, airlines can and will charge fees or require you to buy new round trip tickets if they are able to re-schedule you. Be aware that if you miss your outbound flight the airline will consider you a 'NO SHOW' and cancel all subsequent flights.