The best route is to turn around, get back on the ferry to Dublin, drive down to Rosslare on good roads, get the ferry over to south Wales and then travel along good roads again (much of it motorway as far as I remember) to Cardiff.
The alternative is a scenic but twisty and slow route through the middle of Wales. In short - I don't think there is a 'best way' in Wales. It's not like a nightmare to drive, but it's a long slog over what are effectively old roads through hills, valleys and villages.
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