Ballymachugh is one of the three parishes of the diocese situated in County Cavan. It lies along Lough Sheelin and in it the Diocese (Ardagh and Clonmacnoise) reaches its most easterly point about half a mile from Mountnugent. After the definite establishment of the Diocese of the Ui Briuin, or Kilmore, this parish remained attached to Ardagh because it was part of the old principality of Cairbre Gabhra centred at Granard. This ancient authorities generally speak of Lough Sheelin as in Cairbre; so the book of Lecan, Leobor Gabhala, Book of Leinster. Earlier than the time of Cairbre, son of Niall, these lands bordering Lough Sheelin on the north were inhabited by the pre-Celtic Glasraighe, who were subdued by him; and for whom long afterwards, the genealogists traced a royal descent from Cairbre himself.
At the time of the Christianization of Ireland (c. 5th–6th century) tribal groupings believed to be in or near Breifne included the Glasraighe, Masraige, Dartraige, Armhaighe, Gallraighe, the Fir Manach, and the Gailenga.