This is just to get two of us and the car from dockside Holyhead to dockside Dublin Port. It doesn't count the cost of getting the car to Holyhead in the first place, nor the cost of stashing it somewhere in central Dublin for a few days. So it's pretty expensive, and quite tiring, all things considered.
On the other hand, return tickets for the pair of us from Stevenage to Dublin Port are currently at £152, by train and ferry.
(For comparison, Ryanair from Stansted to Dublin airport would be about £75 by the time they've done their bait-and-switch tactics. Plus parking ... another £50. And then there's the bus from the airport down to the centre. So it could conceivably be cheaper, but not by much.)
I think we may be taking the train this time.
ETA: there's indication on Seat61 that it should be possible to do it for £97 from Royston, assuming we go Stena.