Forggus mac Muirchertaig (died c. 566), also called Forrgus mac Maic Ercae, is included in most lists of High Kings of Ireland.
Father Muirchertach mac Muiredaig, also called Mac Ercae, brother Domnall Ilchelgach with whom he is said to have ruled jointly. Succeeded Diarmait mac Cerbaill, followed by Ainmuire mac Sétnai.
Death recorded in Adomnán's Life of Saint Columba.
The less reliable evidence of the Irish annals records that he and Domnall defeated Eógan Bél, grandson of Ailill Molt, and then, circa 550, Éogan's son Ailill Inbanda further west, in the region of Clew Bay, modern County Mayo. Also fought Diarmait in alliance with Aimmuire.
- Byrne, Francis John (1973), Irish Kings and High-Kings, London: Batsford, ISBN 0-7134-5882-8
- Charles-Edwards, T. M. (2000), Early Christian Ireland, Cambridge: Cambridge University Press, ISBN 0-521-36395-0
- Charles-Edwards, T. M. (2004), "Forggus mac Muirchertaig (d. c. 566)", Oxford Dictionary of National Biography, Oxford: Oxford University Press, retrieved 2008-03-06
- Sharpe, Richard (1995), Adomnán of Iona: Life of St Columba, Harmondsworth: Penguin, ISBN 0-14-044462-9