Erc, son of Dago, is believed to have been a pagan druid who was the only member of King Laoghaire's retinue to pay homage to Saint Patrick during the latter's confrontation with the druids on the Hill of Slane in 433. Dubhthach maccu Lugar also a druid who paid tribute to Patrick and converted. Erc mac Dega was converted to Christianity by Patrick and appointed the first Bishop of Slane. St. Erc’s foundation in Slane had an honourable history for at least six hundred years.
He may have arrived in Kerry soon after the mission of St. Benignus, who was sent by St. Patrick to preach to the tribes of West Munster in 450. This visit of St. Benignus was comparatively short, for he was called away to North Clare and Connaught, where his apostolic labours may have been more urgently needed. To complete the conversion of Kerry, St. Patrick sent Bishop Erc, who had spiritual charge not only of Kerry, but also of a wide range of south-west Limerick, in the heart of which lay the convent of Ita at Killeedy, over which he seems to have had jurisdiction.
Before Saint Patrick died in 461, he sent Bishop Erc southwards to Munster.
Around the year 484, Brigit of Kildare was his travelling companion to his native province.
He was the special friend and tutor of St. Brendan the Navigator, the patron of Kerry. Erc is said to have trained the young Brendan at his church in Ardfert in 512. Erc is also responsible for establishing the famous school at Slane, where King Dagobert II is said to had received his early education.
Dean Cogan, a native of Slane, in the nineteenth century called Erc a man of great sanctity and usefulness. Patrick is reputed to have said, "Bishop Erc – Everything he judged was just; Everyone that passes a just judgement – Shall receive the blessing of Bishop Erc".
In the 16th century, the hill-top monastery became a Franciscan friary supported by the Flemings. 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." Ferta Fear Fiac means "the Graves of the Men of Fiac".
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 held on the 31st of 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, G. H. (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.
Many years later, Erc returned to Slane and lived out his declining years in prayer and solitude in a quiet hermitage beside the Boyne. Erc died on 2 November 514, aged 93 years. His feast in Ireland is held on the 2nd of November.