The personal names Brian and Baron, as is the case with many other personal names, gave rise to surnames. The first was O Briain, a Dalcassian sept, the second gave O Braoin and Mac Braoin. Mac Braoin, according to MacLysaght's Surnames in Ireland, was anglicised to MacBryan and MacBrine in Co Fermanagh, where it is the name of a branch of the MacManus sept.
Anglicised MacBreen, it names a sept of Co Kilkenny, now extinct or indistinguishable from O'Breen. Mac Braoin was anglicised to MacBreen, Mac Bryan and MacBrine, with Mac Brian, O`Breen and Byran, and sometimes to Byrne, as aliases.
Macbreen/McBren is found in The Justiciary Rolls (13081314) 16 times, mainly relating to Cork, Waterford, Tipperary and Limerick. Orla Inion Mhic Bhraoin?
The same year in Limerick, McYertagh McBren, William McBren and Geoffrey McBren McRichard were charged "with divers trespasses and felonies".
In 1314 Donald McBren, Murthut Mc Bren and Mathun Mc Bren, were charged along with "divers other felons to slay and rob John Gothemund", and also "to burn the haggard of Walter Mulcot at Athfade in the county". Athfade from Ath Fada, "the long ford", was later translated to Longford, being the name of a townland in the Co Limerick parish of Oola, in the barony of Coonagh.
Annala Rioghachta Eireann/ Annals of the Four Masters notes the death at the hands of the English in 1381 of Ceindeidigh Mac Brian O gCuanach, as well as their treacherous slaying of Mac Brian O gCuanach in 1408. In 1585 we read of the going into west Connacht of Murtagh Mac Brien O gCuanach, with the last mention by the Masters being to the sons of Mac Brien O gCuanach in 1598.
In a footnote to his 1856 translation of Annala Rioghachta Eireann, John O'Donovan says "the chief of the sept of the O Briens, seated in the Co Limerick barony of Coonagh, took the name of Mac Brien Cuanagh; and the chief of another sept of the same family, seated in the Glen of Aherlow at the foot of the Galty Mountains, in the county of Tipperary, took the name of Mac Brien Aherlagh, while a third branch, seated in the territory of Ara, in the north of the county of Tipperary, took the appellation of Mac-I-Brien-Ara".
Mac Lysaght's Irish Families says the Dalcasian clan, known as Ui Toirdealbhaigh, took the surname O'Brien from Brian Boru. This divided into several branches and possessed a great part of Munster, of which they were frequently kings.
"The O'Briens of Ara (north Tipperary), a territory they acquired from the O'Donegans about the year 1300, had as chief Mac Ui Bhriain Ara; those of Co Limerick gave their name to the barony of Paobblebrien; another branch was located around Aherlow by the Galtees; and another south of the Comeragh Mountains on the rich lands near Dungarvan". No mention here to Mac Briain O gCuanach!
The index compiler for The Irish Fiants of the Tudor Sovereigns (1521-1603) separately lists Mac Brian (with its approximately 130 mentions and various spellings) and MacBryan/ Bryane/Bryen/Bryene. The 40 listings of this latter are found in all provinces, but the greatest number is in Co Limerick. They were at Caherilly, Ballytrasna, Granagh and other unidentified places in Limerick.
The MacByrnes landowners in 1641 outnumbered the Ryans and O Heas in the barony of Coonagh as listed in the Co Limerick Civil Survey (1654-58). They were at Oola and Cluggin, Kilcornane, Templebridane, Doon, Castletown, Kilteely, Grean and Ballinclohie (Bally clogh in the parish of Bruree).
Most of the 38 Mac Breen telephone entries are in the 05 area, but many of the 255 Breen entries (of the 1021 total) in the 06 area must be descendants of Mac Braoin O gCuanach as, no doubt, O'Brien must conceal many originally Mac Breen.