Welsh monastic saint of the 7th century, the centre of whose cult was at Meifod (Powys). Also in this country is the town called after him, Llandysilio, where he built his cell close to the Roman road. Besides the cluster of dedications to him here and in Clwyd, there is a chapel of his picturesquely sited in the Menai Straits, while dedications in former SW. Cardiganshire and another on the borders of former Pembrokeshire and Carmarthenshire (all now in Dyfed) suggest that as well as being a saint of Powys he also travelled to other parts of Wales as a peregrinus. A Tysilio or Suliau, of whom there is a cult in Brittany, is most probably a different person from the Welsh saint, although a Welsh origin is claimed for the Breton saint and a Breton Life makes a lively but unconvincing attempt to combine elements from the biographies of at least two men. Feast: 8 November.
From The Oxford Dictionary of Saints in Oxford Reference.