The Dark Age Celtic Saints aren't strictly Catholic saints, but rather the founding fathers of the church in Brittany, Britain and Ireland. Some documents exist which may be first hand accounts from the period, but most accounts of their actions were composed in the 11th to 13th Century, and mix history, myth, folklore and contemporary church politics1. You can find a growing list of such "Lives" on Mary Jones' site.
The 12th C. Life of Saint Illtud* (Full English Translation; Summary with notes), which was undoubtably based on earlier materials, suggests he was born in Brittany, the son of Bicanus, a warrior, and Rieingulid, daughter of Anblaud, king of Britain (or Brittany?). He married a woman called Trinihid and then left for Wales, where he ran the household of Poulentus, king of Glamorgan. This continued until the rest of the household, while out hunting, were swallowed by the earth for rudely demanding food from St Cadog. After this horror Illtud converted, abandoned his wife, and set up a religious school in Hodnant where he trained the saints Samson, Paulinus, Gildas and David.
He fell out badly with the local king "Meirchion the Wild", who's stewards harrassed him, each feeling the wrath of his god in some way (melting, drowning in mud). At one point Illtud was force into hiding in a cave on the River Ewenny. Finally Meirchion himself came to kill him, and was swallowed by the earth. Under the new-found freedom, more and more visitors came, and Illtud was finally forced to hide once more, this time in order to lead the contemplative life. He hid in a cave at Llwynarth. There he witnessed the wonder, which is also mentioned in the Life2.
Following this he made a return trip to Brittany, where he relieved a famine. He returned to Wales, but travelling back to Brittany in order to die in his homeland, being buried at Dol. The life notes two occasions in which he continued to protect his people through miracles even after his death.
More on floating altars and the body in the boat.