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.
Further source material needed though.