Constantine of Strathclyde

Constantine was reputedly the son and successor of King Riderch Hael of Alt Clut, the Brittonic kingdom later known as Strathclyde. (The modern English name of Alt Clut is Dumbarton Rock.) He appears only in the Life of St. Kentigern by Jocelyn of Furness, which regards him as a cleric, thus connecting him with the several obscure saints named Constantine venerated throughout Britain.
According to Jocelyn, Constantine was the son of Riderch and his queen Languoreth. He succeeded his illustrious father upon his death, but later stepped down to become a clergyman. However, no other sources mention a son of Riderch named Constantine. He is absent from the pedigrees of Northern British kings in the Harleian genealogies and the Bonedd Gwŷr y Gogledd (This is the Descent of the Men of the North). A Saint Constantine was venerated in the area around Glasgow, the setting of much of Jocelyn's narrative; the early church in the nearby burgh of Govan was dedicated to him. However, by the 12th century Saint Constantine's biography was obscure, so it is likely that King Constantine was a literary invention created to provide a narrative for the shadowy early figure.
The compilation of hagiographies in the Orthodox Church known as the "Great Synaxaristes" includes Saint Constantine of Strathclyde, giving his feast day as March 11. It also states that he was guided to Christianity by Saint Columba, became a missionary of the faith in England and Ireland, and died around 640.