Changing ticket booked online on day of travel?

[econgirl]

Hi guys
I am sure this is a stupid question, but I have booked a return ticket Dublin-Limerick with the return portion leaving Limerick at 14:55 - however as I have a feeling I may be able to make the 13:55 train on the day, all going well, I was wondering if I arrive at the ticket office in time, is there any way of exchanging the ticket for one for the earlier train, or because I have booked it online I take it this is not possible and I just have to sit and wait (which is understandable as that's the train I booked in fairness!).

Thanks a million