THE earliest mention of St. David comes from Ireland. It occurs in a short treatise, the Catalogue of the Saints of Ireland, drawn up about A.D. 730. According to this there were "Three Orders" of Irish saints between 432 and 665 ; the "Second Order" began and ended in the sixth century, and we are told that it received a mode of celebrating mass "from holy men of Britain, to wit, from St. David and St. Gildas and St. Docus," or, as another version has it, "from Bishop David, and Gildas, and Docus, Britons." Thus we have early reliable evidence, not only that there was such a man as David, but that he was a notable British saint and bishop, who with two other religious men of Britain, including the celebrated St. Gildas, had agreed upon a form of celebrating Holy Communion, which was widely received among the foremost saints of Ireland in the sixth century. It is therefore more probable than not, that these three Britons, David, Gildas, and Docus, were already men of renown before the middle of that century. From Ireland, too, we have evidence, not quite so early but no less reliable, that St. David's chief monastery was in Mynyw or Menevia, and that his festival fell on the first day of March, for in The Martyrology of Oengus, which is now agreed by all scholars to have been compiled about the year 800, we find under March 1st, "David of the monastery of Mynyw."
A W Wade-Evans, Brother Hermengild TOSF, Paperback, ISBN 10: 1490364331, ISBN 13: 9781490364339