The stone is by the river on the floodplain of the Severn, and not so far from the modern national boundary.
Later on Beuno went to Berriew, in Montgomeryshire, where he was given lands also. But one day whilst there he heard a Saxon shouting to his dogs to pursue a hare on the further side of the Severn, and he at once resolved to leave a place made odious to him, because within sound of the English tongue. In a rage he returned sharply to his disciples, and said, "My sons, put on your clothes and shoes, and let us leave this place, for the nation of this man has a strange language which is abominable, and I heard his voice. They have invaded this land, and will keep it." Then he went deeper into the Welsh land and visited S. Tyssilio, and remained with him forty days.
It sounds like Saint Beuno could get pretty ratty. The next stop he got cross at some young men when he was cooking dinner for them and they got impatient. He cursed one of them, who died the next day. Then there was the episode with Saint Winefred (of the well) - he cursed Caradog for chopping off her head and turned him into a puddle (perhaps fair enough). Winefred wasn't the only person whose free-ranging head he was able to successfully stick back on their neck - he also did it for a princess called Digwg. Her husband had cut off her head, and when her brother found out he chopped off the husband's head too. Beuno sorted him out also, which was quite charitable.
The quote is from volume 16 of Baring-Gould's Lives of the Saints (1914), but for more detail see his Lives of the British Saints (1907).
About a mile east of Berriew, on the green by the side of a lane, is a stone about five feet high, called, on the Ordnance Map, Maen Beuno, but by the people in the neighbourhood "the Bynion Stone." A man who told me (in 1891) that he was fifty years of age, said he had been told by old men when he was a boy that it was intended to have built a church on the spot where the Bynion Stone stands, but that every night the stones which had been placed in position were carried away and put down on the spot where Berriew Church now stands. (1891.)
Scraps of Folklore Collected by John Philipps Emslie
C. S. Burne
Folklore, Vol. 26, No. 2. (Jun. 30, 1915), pp. 153-170.