Counties Leitrim and Cavan formed part of the kingdom of Bréifne, also known as Ui Briuin Breifne, whose septs were descendants of the great Ui Briun clans of Connacht. Following the overthrow of the Conmaicne (Rein) and other ancient tribes about the 8th century, the Uí Ruairc and the Uí Ragallaig were dominant in this region County Leitrim, or West Bréifne, became known as Bréifne O'Rourke, and Cavan, or East Bréifne, became distinctively Bréifne O'Reilly. Bréifne long resisted colonization by the Anglo-Normans, and the O'Reilly's of Cavan were not brought under permanent English rule until the late 16th century. Cavan, previously part of Connacht, was designated a part of Ulster in the early 17th century and included in the Ulster plantation from 1608 onward, when it was settled by Scots and English colonists.

MacBradaigh, MacBrady 

A very ancient and important family in Cavan; they were, according to MacGeoghagan, a branch of the O'Carrolls, Chiefs of Calry.

MacCaba, MacCabe 

A powerful clan originally from Monaghan, but for many centuries settled in Cavan.

MacCagadhain, MacCogan 

Chief of Clan Fearmaighe, a district south of Dartry, and in the present Barony of Dromahaire, county Leitrim. O'Brien states that the MacEgans were Chiefs of Clan Fearamuighe in Brefney: hence MacCagadhain and MacEgan may, probably, have been the same clan.

O'Clery, Clarke 

A branch of the O'Clerys of Connaught and Donegal, and of the same stock as the authors of the Annals of the Four Masters 

MacConsnamha 

(snamh: Irish, "to swim"; anglicised "Ford" or "Forde"), Chief of Clan Cionnaith or Clan Kenny, now known as the Muintir Kenny mountains and adjoining districts near Lough Allen, in the parish of Innismagrath, county Leitrim.

O'Corry 

A clan located about Cootehill.

O'Daly and Mulligan 

Hereditary bards to the O'Riellys.

MacDarchaidh , MacDarcy 

Chief of Cenél Luachain, a district in the Barony of Mohill, county Leitrim, from which the townland of Laheen may he derived.

O'Finnand O'Carroll 

Chiefs of Calraighe or Calry, a district adjoining Dartry in the present Barony of Dromahaire and comprehending, as the name implies, an adjoining portion of Sligo, the parish of "Calry" in that county. O'Carroll: According to the De La Ponce MSS., "O'Carroll" of Calry, has been modernized MacBrady. 

O'Farrelly 

A numerous clan in the county Cavan.

Fitzpatrick 

A clan originally of the Fitzpatricks of Ossory.

Fitzsimon 

A clan long located in the county Cavan of Anglo-Norman descent, who came originally from the English Pale.

MacFionnbhair , Finvar 

Chief of Muintir Gearadhain (O'Gearon or O'Gredan), a district in the southern part of Leitrim.

MacFlannchadha 

(rendered MacClancy), Chief of Dartraidhe or Dartry, an ancient territory co-extensive with the present Barony of Ross-Clogher in Leitrim. O'Ruairc, O'Rourke

MacGiolladuibh, MacGilduff, , Gilduff 

Chiefs of Teallach Gairbheith, now the Barony of "Tullygarvey," in the county Cavan.

MacGobhain, MacGowan, or, O'Gowan 

(gobha: Irish, "a smith"), a name which has been anglicised "Smith," etc., were of the race of Ir; and were remarkable for their great strength and bravery. Thus Smith, Smyth, Smeeth,, and Smythe, may clam their descent from the Milesian MacGowan, originally a powerful clan in Ulidia.

MacMaoilliosa, llison 

Chief of MaghBreacraighe, a district on the border of Leitrim and Longford.

O'Maolmocheirge

The family (followers of Mocheirge) became associated with the Kingdom of Bréifne (Breffny), and in Matheson's 'Special Report on Surnames in Ireland' (1890) 48 with the name Early were found in the Province of Connaught, predominantly in Leitrim (earlier known as West Breffny or Breffny O'Rourke) 

O'Maoilmiadhaig, Mulvey 

Chief of Magh Neise or Nisi, a district, which lay along the river Shannon in the west of Leitrim, near Carrick-on-Shannon. The clans in the counties of Cavan and Leitrim, not given by O'Dugan, are collected from other sources. Several other clans in various parts of Cavan, as O'Murray, MacDonnell, O'Conaghy , Conaty, O'Connell , Connell, MacManus, O'Lynch, MacGilligan, O'Fay, MacGafney, MacHugh, O'Dolan, O'Drum, etc

O'Raghailaigh ,O'Reilly 

The princes of the territory of Brefney.

MacRaghanaill , MacRannall 

(angilcised Reynolds), who were Chiefs of Muintir Eoluis, a territory which comprised almost the whole of the present baronies of Leitrim, Mohill, and Carrygallen, in the county Leitrim, with a portion of the north of Longford. This family, like the O'Farrells, princes of Annaly or Longford, were of the race of Ir or Clan-na-Rory; and one of their descendants, the celebrated wit and poet, George Nugent Reynolds, Esq., of Letterfian, in Leitrim, is stated to have been the author of the beautiful song called "The Exile of Erin," though its composition was claimed by Thomas Campbell, author of "The Pleasures of Hope."

The Mac-Samhradhain 

(anglicised MacGauran, Magauran, and Magovern) were Chiefs of Teallach  Eachach (which signifies the tribe or territory of Ecchy), now in the Barony of "Tullaghagh," county Cavan. This surname is by some rendered "Somers," and "Summers," from the Irish word "Samhradh" [sovru], which signifies "summer".

O'Sheridan 

An ancient clan in the county Cavan. Richard Brinsley Sheridan, one of the most eminent men of his age, as an orator, dramatist, and poet, was of this clan.

MacTaichligh, or, MacTilly 

Chief of a district in the parish of Drung, in the Barony of Tullygarvey.

Mac-Tighearnain 

(tigherna, Irish, "a Lord or master"), Anglicized MacTernan, McKiernan, and Masterson, were Chiefs of Teallach  Dunchada (signifying the tribe or territory of Donogh), now the Barony of "Tullyhunco," in the county Cavan.

And several clans in the county Leitrim, not mentioned by O'Dugan, as MacGloin, of Rossinver; MacFergus, who were hereditary erenachs of the churches of Rossinver, and whose name has been Anglicized "Ferguson"; O'Cuirnin, or Curran,, celebrated bards and historians; MacKenny, or Keaney, MacCartan, O'Meehan,, etc