O'Connor or O'Conor, is perhaps the most illustrious of all Irish surnames, though this view would, no doubt, be disputed by the O'Neills, the O'Briens, the O'Donnells and one or two other great and famous septs. It is borne by six distinct septs located in different parts of the country of whom four survive in considerable numbers. The most important are the O'Connors of Connacht - the main branches of this sept being O'Conor Don, O'Conor Roe and O'Conor Sligo. These are descended from Conchobhar, King of Connacht (d. 971), and the last two High-Kings of Ireland were of this line, viz., Turlough O'Connor (1088-1156) and Roderick O'Connor (1116-1198), both of whom were progressive monarchs. Their direct descendant, as certified by the Genealogical Office, Dublin Castle, is the present O'Conor Don: Denis O'Conor, and it is interesting to note that this important and aristocratic family consistently maintained its position not-withstanding the fact that they remained flexibly Catholic. Evidence of this is abundant in all the sixteenth, seventeenth and eighteenth century manuscripts.

In dealing with the landed proprietors of Conacht, among the most distinguished members of the O'Conor Don stock four O'Conors of Belnagare are outstanding in the field of culture: Charles O'Conor (1710-1791), antiquary and collector of Irish manuscripts; his two grandsons, Rev. Charles O'Conor, D.D., P.P. (1764-1828), librarian at Stowe and author inter alia, of Rerum Hibernicarum Scriptores Veteres and Mathew O'Conor (1773-1844), author of History of the Irish Catholics etc; and Charles Owen O'Conor, O'Conor Don (1838-1906), President of the Royal Irish Academy and of the Society for Preserving the Irish language and author of The O'Conors of Connacht. In the miltary sphere Cabrrach O'Conor (1584-1655) and Hugh O'Conor (d. 1669), respectively son and grandson of O'Conor Don, took a prominent part in the 1641-1652 wars. Three of this sept were outstanding in the Irish Brigade. More recently, one of the Roe branch, General Sir Luke O'Connor (1832-1915), who had enlisted as a private soldier in the British army, won V.C. and a commission for his remarkable bravery at the battle of Alma.

O'Connor Kerry, as the chief of the Munster O'Connors was called, derives his name from a different Conchobhar. He was lord of an extensive area in north Kerry, but after the invasion of 1170 Anglo-Norman pressure pushed the O'Connors northwards towards the Shannon estuary. However, they still retained a considerable territory, in fact the greater part of the modern barony of Iraghticonor, which is an attempt at a phonetic spelling of Oireacht ui Chonchobhair, i.e. O'Connor's district of government: their chief strong hold in Iraghticonor was Carrigafoyle Castle. From this spet came a number of distinguished officers of the Irish Brigade in France, the best known of whom was Arthur O'Connor (1761-1834), an erratic character who was also a member of the United Irishmen, and the latter's son, Fergus O'Connor (1794-1855), the chartist. Some of this family changed their name to Conner. The three most notable Irish-American O'Connors were of this sept: the brothers Michael O'Connor (1810-1872), and James O'Connor (1823-1890), both Catholic bishops in U.S.A., and Patrick Edward O'Connor (1820-1871), pioneer, Indian fighter and soldier in the Civil War on the Confederate side. The O'Connor sept of Kerry is at the present day much the most numerous of them all. It is estimated that there are almost 30,000 persons of the name in Ireland to-day - it comes ninth in the list of commonest surnames and the vast majority of these are from Kerry or from the adjoining counties of Cork and Limerick.

The O'Connors of Corcomroe, a barony in north Clare on the shores of the Atlantic, are stil extant. The eponymous ancestor n this case was Conchobhar, lord of Corcomroe (d. 1002).

The fourth of the surviving septs was O'Connor of Offaly. O'Connor Faly, as the chief was called, was a royal descent, his ancestor being Cathaior Mor, King of Ireland in the second century. The eponymous Conchobhar in this case was much later than Cathaoir and belongs to historical times as he died in 979. This sept was constantly engaged in war with the invader until the middle of the sixteenth century when they were vanquished and dispossed of most of their estates. They were still in Offaly in 1689, as Col. John O'Connor was member for Philipstown in King James II's Parliament and they were represented by the family of O'Connor-Morris of the same county until quite recently.

It should be added that there was also a powerful sept of O'Connor in Keenaght (Derry), which in the twelf century was overpowered by the O'Kanes. They are mentioned here because, though as a sept they were eliminated, families of O'Connor are still found in that part of Ulster and it may be assumed that they are descended from the once famous O'Connors of Glengiven who were of royal blood, their ancestors being Cian, son of Oilioll Olum, King of Muster in the third century.

The history of the O'Connors, particularly, those of Connacht, forms the subject of a number of books which can be consulted for detailed information concerning these important septs.