I know Wales are away to Ireland on Sunday - would it be expected trains out to cArmarthan around 1600-1900 on Saturday would be rammed wig supported going to Ireland by ferry?
I've no idea whether fans might travel early and hang around at the port, but the trains between 16:00 and 19:00 don't connect with ferries. The daytime boat train to Fishguard leaves Cardiff arround 11am and the FirstGWR journey planner is telling me the night boat train on Saturday evening / Sunday morning is replaced by road transport from Carmarthen (train from Cardiff to Carmarthen leaving at 22:07).