Ffynon Fair (St. Mary's Well). -- This is a spring and well near the church. It was a holy well and under the guardianship of the Virgin. Its water was used for the sacred uses of the Church, as in the font for baptism, etc. , and in years gone by it was held in great repute for its curative properties, especially as a bath for rheumatic complaints and cutaneous disorders. It was formerly held also to have talismanic properties as a protective against curses, witchcraft, and other evils of life. But what was the superstitious cantrap necessary to be used to make it efficacious is not now known.
This is from 'A parochial history of Llanfair Caereinion' in 'Collections Historical and Archaeological relating to Montgomeryshire and its borders' (1868). You can read about the several other wells in the area and their curative powers.
Coflein suggests that the well has been "claimed" as pre-Christian and to have curative powers:
St. Mary's Well is located at the bottom of a flight of concrete steps leading from the churchyard of St. Mary's Church (NPRN 408480) to the river Banwy below, but almost certainly pre-dates the church; it has even been claimed to have had pre-Christian origins. The well has long been believed to have had curative powers, and was restored with funds from the Prince of Wales Trust in 1990, and set within an enclosed garden.