Train stuff in Belfast worked perfectly thanks.
It may be of interest to know that the car park at Connolly has one pay and display machine that doesn't take credit cards (near the top of the car park) and the one at the bottom, near the attendant's hut has the credit card bit out of action. (it was raining so I didn't survey the one I could see way over in the distance) Had to get change from the attendant, who didn't seem to understand the question when I asked if paying for 3 days (€21) would be ok when I would be a couple of hours past the time.
I still don't know if you can pay for part of a day beyond a full day, or if they only take for full days once you are paying for a few days. In the end I paid for 3 and risked it rather than pay €7 for 2 hours. As it happened I caught an earlier train home and only overran by about an hour.