Okay so I've checked out prices and it's €96 for a return ticket from Rosslare to Swansea (this is via Stenaline and their rail-and-sail offer). Basically, make your way to Rosslare then grab a ferry at 9am then you arrive in Fishguard at 12.30pm then grab a train (at 1.27pm) from there to Swansea. There is one transfer in Llanelli and you arrive in Swansea at 3.22pm.
Anyone up for it? I've emailed Arriva asking how many bicycles are allowed per train so hopefully I'll hear back from them soon