One generally has to be sympathetic to the complaints of the faithful when well-beloved saints are thrown out of the window. Assuredly the benefit of the doubt should rest upon the cultus that has arisen around such personalities, and it may seem unreasonable to methodologically reject the possibility of supernatural inspiration or intervention when it comes to the saints being glorified by the Church; secular scholarship may have no access to the history of St. Philomena, but as a spiritual practice it would seem strange to reject out of hand the revelations of her life and the involvement of the Cure of Ars in the promulgation of her cultus.
Nonetheless, not all saints did in fact exist, and as a cautionary tale it may be helpful to read the life of a saint whom modern sensibilities might baulk at. For this saint, St. Muirghein the Meer (pronounced Morien), was in fact according to the legend a mermaid. Even a mind reinforced by a hearty dose of naive traditionalism might find this story just a tad incredulous.
This story comes from the Lebor na h-Uidre (The Book of the Dun Cow), an Irish manuscript dating from the late 11th century, and whose compiler is said to have died in 1106. The legends and traditions recorded are undoubtedly of much older provenance; this one comes from the Aided Echach mheic Mhaireda (Destruction of Eiochaid mac Mairidh). It was translated into modern English in Silva Gadelica, II, 265-269, and again by Crowe, Proceedings of R. H. and A. A. of Ireland, 1870, 94-112, and also Joyce, Old Celtic Romances, London, 1879, 97-105. I copied it from Lucy Allen Paton, Studies in the Fairy Mythology of Arthurian Romance, New York, 1960, pp. 9-10.
A certain well in Ulster overflowed its banks. Liban, the daughter of Ecca, king of Ulster, was the only member of her race who escaped death in the flood. She was transformed into a salmon below the waist, and with her pet dog, which had been changed into an otter, she passed three centuries in the waters of the lake (Lough Neagh) which had been formed by the overflowing of the well. At the end of this time, St. Comgall of Bennachar, or Bangor, sent Bevan mac Imle on a mission to Gregory. As Bevan sailed over the sea he heard a chanting as of angels in the waters beneath him, and when he asked whence the song came, Liban replied that she made it, and forthwith told him her story, adding that her purpose in coming had been to bid him keep tryst with her a year from that day at innbher Ollorba. At the appointed day and place the nets were made ready, and the mermaid was taken in the net of a certain Fergus. "She was brought to land, her form and her whole description being wonderful. Numbers came to view her and she in a vessel with water all about her." Soon a contest for her possession arose among Comgall, Fergus and Bevan. By fasting they won a revelation from heaven which bade them yoke to the chariot in which Liban was placed two stags, and allow them to go with her wheresoever they would. The stags bore Liban away to tech Dabheoc. Then the clergy gave her her choice of being baptized and going to heaven in the fulness of time, or of living on earth for three hundred years. "The election she made was to depart then. Comgall baptized her, and the name that he conferred on her was Muirghein or 'sea-birth' as before; or perhaps Muirgheilt, i.e. 'sea prodigy,' that is to say geilt in mhara, or 'the prodigy of the sea'." Liban Muirghein was afterwards worshipped as a saint at the town of Tec-da-Beoc.
Lucy Allen Paton, in the study from which that except was copied, ties the legend of St. Liban Muirghein in with a broader tradition that gave arise to the Arthurian figure of Morgain la Fee. The Arthurian figure's connection with the sea does not appear until the romance of Floriant et Floirete in the 13th century, in which she lives in Sicily, a fact verified by other local traditions pointing to a Sicilian origin to the story of Morgain's aquatic origins. Instead of seeing a direct genealogy from the mermaid saint to Morgain la Fee, Paton suggests that Morgain is a conflation of St. Muirghein with the Irish battle-goddess Morrigan.