In sickness fare to Islay

In 1912 (June 22), the British Medical Journal published a letter entitled 'Medical Men In The Hebrides' by J. A. Goodchild reflecting on his time as a doctor in Iona at the turn of the 20th century. In it he mentions 'an old Gaelic saying' that refers to Islay as the first port of call for the Hebridean sick:

'The Mull doctors, one of whom has a fine Gothic tomb stone in the Reilic Oran at lona, were long looked on as the first in Scotland. These were the Beatons, who succeeded each other from time immemorial, and were occasionally summoned to Edinburgh to attend royalties. A daughter of one of them was amongst the four Maries of the old song. The old herb garden in which they grew their simples still keeps its name in the south-east of Mull, and it looks as if the only other medical attendant formerly available in the Hebrides was in Islay, if one may trust an old Gaelic saying : "In sickness fare to Islay, if Islay fails try Mull, if Mull says, 'I canna,' the de'il has ye''.

Not sure what period this saying dates from, as for the Beatons they practiced medicine in the Western Isles from the Middle Ages down to the 18th century.