Online fare promotion has been cut back for Christmas
1. The 3+ day advance fare is on longer available, the 1-2 day fare now applies all bookings made a least a day in advance till Jan 2nd
2. Family offer
http://www.irishrail.ie/cat_offers.jsp?i=4737 is probably the cheapest option if traveling as a group
3. Bookings made on/before December 17th offered the 3+ day advance fare, so lesson as always to book in advance