Marked on the O/S map as Tumulus, I saw this site mentioned on Megalithic.
Not far from the St Fagans Museum of Welsh Life which is an excellent place – well worth a visit (free entry) and has a replica Iron Age village and wooden circle/henge - amongst many other fine buildings.
Take the minor road west out of St Fagans to St Brides Super Ely. Just before entering the village take the turning on the right signposted Parc Coed Machen Country Cottages (brown sign). Follow the narrow lane uphill and park near Pen Onn / Sant Y Nyll houses. Opposite the houses is a public foot path which leads to a pretty ruined windmill (5 minute walk) – easily seen across the field. Once you get to the windmill, go through the field gate below it (near the trees and other ruined building) and go into the next field. The Barrow is on the high point to your left.
The Barrow itself is 20 paces across and 0.5 metres in height.
It is strange how on the one side of the Barrow was a tranquil ivy covered ruined windmill and on the other a busy, noisy main road (A4232). I prefer the windmill side!
Easy site to access but very muddy walk in places.