We are just 2Hr 30 Mins from the M4 bridge over the Bristol Channel and 2 hours beyond Cardiff.
The most straightforward route is to remain on the A40 westbound at Carmarthen which is dual carriageway all the way to St Clears (there is a good service area at the end of the dual carriageway if you need a cup of tea). Continue on a further 10 miles until you come to the roundabout for the A 478 Tenby to Cardigan road. Turn right there and continue all the way to Cardigan.
At the roundabout just to the south of Cardigan, take the immediate left turn on to the A487 towards Fishguard, and just as you reach the top of the hill you will see a sign for Fish and Chips. Turn right down that road (Longdown Road but no signs) for about half a mile. We are on the right hand side just after the dip in the road.
If you miss a turning and find yourselves in St Dogmaels village, turn up David Street and then second left on Longdown Bank. Plas Newydd is located on the brow of the hill on the left side of the road.
If the weather is fine, a shorter route from Carmarthen (but not in time) is the A484 and B4333 via Newcastle Emlyn. It is picturesque but twisty in places and best avoided in darkness or bad weather.