From the comments box at "Ecclesia Anglicana blo:
Other saints have been referred to as ‘Mermen’ in historical literature of the British Isles. Some scholars hold that it meant one who dwelt on the smaller isles off the coast (the same that were the choice of ‘desert’ for British, Irish, and French monastics.) So, a ‘Mer-man’ or ‘Mer-maid’ was someone who lived on the sea. Those small islands like Iona, Caldey, or Skellig Michael were not all that accessible - it was really ‘living out upon the sea/in the sea’. The local churches saw these islands as ‘halfway to heaven’ - they were dangerous, ideal for ascetic labour, and pre-Christian belief was that such were the abodes of the Dead (see the ‘Blessed Isles’ tradition.) We know archaeologically that many islands off of Britain were used as graveyards for that same reason. No doubt embellishment of the stories mixed with borrowed ‘Classical Roman’ ideas such as ‘half man, half fish’ mermaids produced the later form of the stories. St. Murgen, however, I would bet to have been really a castaway on one of those islands: living a natural hermit-like existence.
Ah, ha! So maybe she does exist after all!
But then, how do we explain this:
In 1560, a team of Jesuits aided by the adjutant of the Portuguese Viceroy of Goa–a physician named Bosquez–dissected seven mermaids washed up on the coast of Ceylon and concluded in their report to the Society that they were anatomically identical to humans and had similar souls, which would have been a great comfort to the siren of Iona. The Church was eager to ascertain at the time exactly what these creatures were, whether they had souls or not, as if we encountered space aliens today, but no official ruling ever came down and the matter became something of a moot point.