Bryn Myrddin is located 2.1 miles up an unnamed lane from the A40 at Pontargothi. From the M4 westbound towards Carmarthen, take the A48 after the motorway ends. Then look for the turnoff signposted to the National Botanic Garden of Wales on the B4310. Follow this road to Nantgaredig, crossing the B4300. At the junction of Station Road and the A40, turn right. You will enter the village of Pontargothi. Just before the bridge, and opposite Cothi House, take a left turn up the unnamed lane. Follow this lane for 2.1 miles, passing Holy Trinity church. Keep turning right at each junction. You will pass Fferm Y Graig and then Plas Altyferin. Bryn Myrddin is a few hundred metres further on the left.
This is a zoomable Google Map
And the map below is a fixed map showing the distance and direction from the A40.