According to a traditional Irish pedigree, he was Piliste, the son of Pherine, a descendant of Noah's son Japheth through Magog and Nemhidh. See John O'Hart, Irish Pedigrees: Origin and Stem of the Irish Nation (reprinted 1989), 43, citing "MacFirbis's Genealogies".
According to the imaginative reconstruction of David Hughes, he was Jasna "Piliste", a son of Jesus' brother Jose(ph). See David Hughes, The British Chronicles (2007), Vol. 2, Table 27A; see also Brian Starr, The Life of Saint Brychan (2008), 113-14. However, there is no contemporary evidence of the relationship.