This sounds great Aran and Dee, and Carmarthen should be accessible for people.
Some thoughts- this is likely to be in the evening rather than afternoon?
Wales are probably playing South Africa in Washington that day, so if that kicks off before the party then people may want to gather in a bar beforehand to watch that.
The venue, Halliwell Centre, on University of Wales Trinity Saint David's campus, is a 25-minute walk from Carmarthen train station, or a short taxi ride if several people arrive on a train at similar times and want to split the cost.
Looking forward already- thanks Dee!