In a field called Croeslechau about two miles eastward of this town or village [Talgarth] but in the parish of Bronllys and on a farm called Bryn-y-groes, is a cromlech, not merely interesting on account of its antiquity, but from the circumstance of a white thorn growing close, and indeed under part of it, which has gradually raised the horizontal or covering-stone several inches out of its original position; it is therefore not only venerable as a relic of very ancient days but as a natural curiosity.
Theophilus Jones, History of the County of Brecknock, v2, 1809.
The RCAHMW’s 1986 inventory of ancient monuments in Brecknock puts the site 500m south west of Pontithel, and includes a description by Edward Lhuyd from about 1700.
In Ireland surely a barrow with a strange hawthorn (white thorn) would have been given a wider berth... an indication that the fairies were living there and wouldn’t be happy about any disturbance. But maybe things don’t work that way in Wales. The barrow was destroyed in the first part of the 19th century and it’s not very obvious where it was.