Thank you for directing me to Frances' site - I have always believe Santon to be St Anne, and the following quotation from Frances' pages explaines:
A very ancient Celtic manuscript states that the 'Hymn of Sanctain' is one of the oldest Irish manuscripts. In the Calendar of Aengus, the phrase 'Epscop Santain sochla' ( the famous Bishop Sanctain) is used. There were several Irish Keeills which were also dedicated to St. Sanctain.

Some time in the seventeenth century, in post-Reformation times, it would appear that St. Santain's connection with Santon had been forgotten, for at that time there took place an even further corruption, which was quite erroneous, when the dedication was mistakenly attributed to St. Ann. How this occurred is lost in antiquity, but it is not difficult to appreciate the easy way in which Santan could be mistaken for St. Ann, and vice versa. Blundell perpetuated this error in his account of the island in 1648. It is interesting to note that in addition to Santon Church, at least one Irish church had its name similarly altered in error, for Kell Easpoig Sanctain, near Dublin, was changed to St. Ann's Chapel.