A friend of mine travelled up on RYR to DUB 2 weeks ago, 14 on the flight...
I do wonder about it.
Once taxes have been taken into account, if you want to go to Dublin city centre or the Southside of Dublin, they are not competitive with the train service on price, frequency or time.
So, their market is people travelling to the Northside of Dublin and connecting passengers at Dublin airport. But, of course, Ryanair don't offer interlining...
It seems a pretty limited market to fill a 737-800.