Erc mac Dega (Latin: Ercus; Cornish: Erth; also known incorrectly as Herygh) was an Irish saint who was apparently also active in Cornwall.
Erc was the only person to give homage to Saint Patrick during the latter's confrontation with the druids on the Hill of Slane in 433. Patrick later ordained him a priest and bishop of Slane. Erc is said to have trained the young Saint Brendan the Navigator at his church in Ardfert. Saint Erc is also responsible for establishing the famous school at Slane, where King Dagobert II is said to have received his early education. In the grounds of Slane Castle are the ruins of St Erc's Hermitage. This consists of a late fifteenth or early sixteenth century chapel and an earlier dwelling. The 12th century martyrology of Gorman calls him "Erc of Slane, bishop of Lilcach and from Ferta Fer Feic beside Sid Truim from the West." His feast in Ireland is 2 November.
The Cornish Saint Erc is generally, though not certainly, believed to be the same man. He was the brother of Saints Uny and Ia and crossed from Ireland to Cornwall, where a church and the village of St Erth are dedicated under his patronage. His feast in Cornwall is 31 October. Little is recorded of him apart from what William of Worcester wrote in 1478: "Saint Herygh, the brother of Saint Uny, a bishop, lies in a certain church situated under the cross of the church of Saint Paul in London; his day is kept on the vigil of All Saints, that is, the last day of October ... Saint Hya ... the sister of Saint Herygh ..." (quoted in Doble (1960)). (The statement about St Paul's may be due to a mistaken identification with St Erconwald.) At Trevessa in the parish of St Erth was a chapel of St Ercus in 1403.