PEIRIO was a son of Caw. He is also represented as a son of Gildas, and therefore grandson of Caw and to have founded Llanfair y Mynydd, now called S. Mary's Hill, near Bridgend, in Glamorganshire. He entered the congregation of S. Illtyd, and, according to the lolo MSS., succeeded him as principal of Llantwit, where he lies buried.
These notices, it should be mentioned, are late. Philologically, it would not be possible to equate Peirio with the Piro or Pirus who occurs in the Life of S. Samson as head of Ynys Pyr or Caldey Isle. The island monastery would seem to have been originally Llan Illtut, and Llantwit accordingly was Llantwit Major. But the earlier name fell into oblivion and was replaced by that of Ynys Pyr (or Byr) . The term Insula is variously applied to an actual island and to a monastic settlement on the mainland ; so that when, as in the Vita Gildae we read : " Quae insula usque in hodiernum diem Lanna Hilduti vocitatur," it is uncertain whether Caldey or Llantwit is meant. The Vita Samsonis says, however : " Erat non longe ab hoc monasterio (i.e. Hilduti) insula quaedam nuper fundata a quodam egregio viro ac sancto presbytero, nomine Piro."
Whether Piro, or Pirus as he is called in the Book of Llan Daf, be the same as Peirio, brother or son of Gildas, is uncertain ; probably not. Piro, as head of Llan Illtut in Caldey, was not quite the right man for such a position. He got so drunk one night that, in returning to his cell, he tumbled into the well, and was pulled out dead. After this catastrophe S. Samson was elected head to replace him, but the rule under Piro had been so lax, that Samson found it impossible to bring the young monks into discipline, and threw up the abbacy in disgust. Rhosbeirio, subject to Bodewryd, Anglesey, is dedicated to the memory of Peirio. Leland calls it " Bettws Rosbeirio."
In the Record of Caernarvon, land at " Rospyriaw " is mentioned as being held " de Sco Birryow."