You are not supposed to buy upgrades from web fares (nor can you buy upgrades from advances in the UK in general, except on certain train operating companies on weekends).
http://www.irishrail.ie/your_ticket/fares_enquiries.asp says
Quote:
|
These fares are standard class fares only and are not applicable in conjunction with premier class travel when making a booking. These promotional fares are not available at a booking office - full single fare will apply.
|