According to legend, once Sithney (or Sezni) made his way to Brittany, God asked him to become the patron of girls looking for husbands. This task struck Sithney as over-daunting and he begged to be allowed to be patron of mad dogs instead. God said that was fine and to this day mad dogs drink from Sithney’s well, and are cured.
If you consult Baring-Gould’s Lives of the British Saints, or Butler’s Lives of the Saints, which I have, you will find no reference to this legend, but you will read how Sithney castigated and cursed a woman and her child because she was washing linen in the town fountain, which doesn’t especially saintly to me. The Bretons also claim that Sithney’s relics (bones, assorted body parts) were responsible for so many miracles that the Irish sent a fleet to steal them and carry them away. And did I mention that he lived to be 127 years old?