O'Caithniadh (or O'Catney)	Chief of Iorras, now the Barony of "Erris," in Mayo
O'Caomhain	(see the "Cowan" pedigree), a senior branch of the O'Dowd family, and Chiefs of some districts on the borders of Sligo and Mayo, in the baronies of Tireragh, Corran, and Costello
O'Ceallachainor O'Callaghan	Chiefs in Erris; this family was not of the O'Callaghans of Munster
O'Ceirin or O'Kearns	Chiefs of Ciarraighe Loch-na-Nairneadha territory in the Barony of Costello, county Mayo, comprising the parishes of Aghamore, Bekan, and Knock
O'Ciernachain	Kernighanand O'Huathmharain (O'Horan or O'Haran), other Chiefs in Lieney
O'Conniallain or O'Connellan	Chief of Bun-ui-Conniallan, now "Bonny-connellan" -- a district in the Barony of Gallen, county Mayo; and also of Cloon-connellan, in the Barony of Kilmain
O'Dubhalen or O'Devlin	Another Chief in Corran
MacDiarmada or MacDermott	Chief of Tir Oliolla, now the Barony of Tirerill, in the county Sligo. The MacDermotts were also princes of Moylurg, in the county Roscommon, in South Connaught. They afterwards became princes of Coolavin, as successors to the O'Garas, Lords of Coolavin; and to the present day, as the only family of the Milesian Clans who have preserved their ancient titles, retain the title of "Prince of Coolavin."
MacDonchaidh or MacDonogh	A branch of the MacDermotts, afterwards Chiefs of Tirerill and of Corran, now the Barony of "Corran" in Sligo. O'Donchathaigh is given by O'Dugan as a Chief in Corran; this name has been anglicised O'Donogh
O'Gadhra or O'Gara	Given by O'Dugan as Chief of Lieney, but in aftertimes Lord of Cuil-O'bh-fionn, now the Barony of "Coolavin," was of the same stock as the O'Haras and O'Briens, kings of Thomond
O'Gaibhtheachain or O'Gaughan	and O'Maoilfhionaor O'Molina, Chiefs of Calraighe Moy Heleog -- a district comprising the parish of "Crossmolina," in the Barony of Tyrawley, and county Mayo
O'Gairmiallaigh	or O'Garvaly, and O'Dorchaidhe or O'Dorchy, Chiefs of Partraigh or Partry; an ancient territory at the Partry Mountains in Mayo, the situation of which the present parish of "Party" determines
O'Gormog	(modernizedO'Gorman), another Chief in Carra
O'Headhra or O'Hara	Chief of Luighne, now the Barony of "Lieney" in the county Sligo; but Lieney anciently comprised part of the baronies of Costello and Gallen in Mayo. The O'Haras were descended from Olioll Olum, King of Munster in the third century. In the reigns of Queen Anne and George the First, the O'Haras were created "Barons of Tirawley and Kilmain," in the county Mayo
O'Lachtnain or Loughnan	(by some of the family anglicised "Loftus"), Chiefs of the territory called "The Two Bacs," now the parish of Backs, situated between Lough Conn and the river Moy, in Mayo
O'Maille or O'ley	Chief of Umhall, which O'Dugan states was divided into two territories

Of every weather ye are prophets;
A tribute of brotherly affection and of friendship."

O'Maolcluiche or Mulclohy	(cloch: Irish, "a stone"), Chief of Cairbre, now the Barony of Carbery, in the county Sligo. This name has been Anglicized "Stone" and "Stoney."
O'Maolfoghmair	Anglicised "Milford" and O'Maolbrennain, anglicised "Mulrennin," Chiefs of Hy-Eachach Muaidhe, a district extending along the western bank of the river "Moy," between Ballina and Killala
O'Monganor O'Mangan	Chiefs of Breach Magh -- a district in the parish of Kilmore Moy, on the eastern bank of the Moy, in the co. Sligo
O'Muiredhaigh or O'Murray	Chief of Ceara, now the Barony of "Carra," in the county Mayo; and also Chief of the Lagan, a district in the northern part of the Barony of Tirawley, in Mayo
O'Talcharaln	Chief of Conmaicne Cuile, now the Barony of Kilmain, co. Mayo. The following Chiefs and clans, not given in O'Dugan, have been collected from other sources
O'Tighearnaigh or O'Tierney	A Chief in Carra

other clans in Mayo and Sligo were:

O'Bannen
O'Brogan
Mac Conbain
O'Bean	(ban: Irish, white), some of whom have anglicised the name "White" and "Whyte;"
O'Beolan or O'Boland
O'Beirne	some of whom have anglicised their name "Barnes;"
O'Flatelly
O'Crean
O'Carey
O'Conachtain or O'Conaty	of Cabrach or Cabra in Tireragh
O'Flanelly
O'Coolaghan
O'Burns
O'Hughes
O'Huada or Heady
O'Fuada or Fodey	(fuadach: Irish, an elopement), and O'Tapa or Tappy (tapadh: Irish, haste) -- these three last surnames have been Anglicized "Swift;"
O'Loingsy or O'Lynch
O'Maolmoicheirghe	(mock: Irish, early), anglicised "Early" and "Eardly;"
O'Mulrooney or Rooney
O'Moran
O'Muldoon
O'Meehan
O'Caffrey or Caffrey
O'Finnegan
O'Morrisey
O'Morris or O'Morrison
MacGeraghty	anglicised "Garrett;" O'Spillane
O'Donnell
MacSweeney