Breifne is said[1] to derive from an obsolete Irish word meaning "hilly", a description which describes the topography of this part of Ireland.

Alternatively, the Metrical Dindshenchas states the name is derived from Brefne, daughter of Beoan mac Bethaig, the grandson of Nemed, a brave soldier-woman.

[5][6] In ancient times the area that became known as Bréifne was said to be occupied by the Erdini, called in Irish 'Ernaigh', who possessed the entire country bordering Lough Erne.

At the time of the Christianization of Ireland (c. 5th–6th century), groups believed to be in or near Breifne included the Glasraighe, Masraige, Dartraige, Armhaighe, Gallraighe, the Fir Manach, and the Gailenga.

During the 12th century the reign of Tighearnán Ua Ruairc, the kingdom of Breifne was said to comprise most of the modern counties of Leitrim and Cavan, and parts of Longford, Meath, Fermanagh and Sligo.

Duncarbry (Dun Chairbre) marks the border of Cairbre's territory on the Drowes, while the Barony of Carbury in North Sligo still reminds us also today.