There were two great septs of O Cathain. The earlier anglicized form of this name was O'Cahan, and even as late as the beginning of the present century, O'Cahans were still found in Co. Derry: but in modern times the forms Keane, Kane, and sometimes O'Kane, are almost universally used, Keane in Munster and Connacht, Kane in Ulster. The two septs were quite distinct originally, but if the belief that the Keanes of Thomond are a minor branch of O Cathain of Ulster is true, as the best authorities asserts, the propinquity of Clare to Galway must necessarily lead to uncertainty in the west of Ireland in cases where no pedigree or reliable family tradition exists. In this connexion it should be added that the Cahanes of west Clare, who were corabs of St. Senan, wrote their name MacCahan and are thought to be quite distinct from the O'Cahanes.

The O'Kanes of Keenaght and Coleraine (Co. Derry) were a powerful and important sept, though not of much account before the twelfth century when they ousted the O'Connors of Glengiven (mod. Dungiven) from their territory. Once established there they retained their ascendancy in the country which is now Co. Derry until they were ruined by the Plantation of Ulster. Many of this sept appear in the Annals from the year 1170 onwards. According to Keating, O'Cahan was one of the inaugurators of O'Neill. In 1598 the last of their regularly inaugurated chieftains, Donnell Ballagh O'Cahan (d. 1617) was formally installed as such. He joined Tyrone (O'Neill) in the great war against the English but later, having submitted with the loss of much of his estates, he so far changed his allegiance as to be knighted by James 1; Nevertheless, he spent his last years as an untried prisoner in the Tower of London. A century later, Sir Richard Kane (1666-1736) distinguished himself as a soldier in the British army and as a writer on military subjects. Echlin O'Kane (1720-1790), an Ulsterman though born in Drogheda, was one of the most famous Irish harpers of the eighteenth century, playing as he did in several European courts. Sir Robert John Kane (1809-1890) was a leading Dublin scientist, best remembered for his book The Industrial Resources of Ireland. O'Donovan says he was of the Derry sept. The MacCloskeys of Co. Derry are a branch of the O'Kanes, being descended from Bloskey O'Kane, slayer of Murtagh O'Loughlin - heir to the throne of Ireland in 1196. The best known of these in modern times was Dr. MacCloskey, Archbishop of New York, who was created Cardinal in 1875. Another branch became MacEvinney or MacAvinny (Mac Aibhne in Irish), the eponymous ancestor being Aibhne O Cathain. It must be remembered, however, that MacEvinney is also the anglicized form of the Breffny surname Mac Dhuibhne. Apart from the prowess of the O'Cahans in Ulster in medieval times, the Thomond O'Cahans, or O'Keanes as they were usually called on the continent, supplied many distinguished officers to the armies of France and Spain in the eighteenth century, notably Eugene O'Keane (killed in action 1693), one of fourteen brothers four of whom served in France.

This name has become corrupted to Kyan in Co. Wicklow where, according to Edmund Hogan S.J. and other authorities, a leading branch of the O'Cahan sept of Co. Derry were settled in the eighteenth century at Ballymurtagh. Of this family was Esmond Kyan who was executed (half an hour before pardon arrived) for his part in the 1798 Insurrection. He was one of the few insurgents with previous military experience. The name Kyan is quite distinct from Kyne.

The other sept was of Ui Fiachrach, located in Co. Galway. Though numerous they were not of great prominence in Connacht in the history of the province, where, however, they are still to be found in large numbers, usually called Cain or Cane.

Finally, the name Kean, usually nowadays without the final e, is that of a Co. Waterford sept quite distinct from O'Cahane, the surname being O Cein in Irish. This sept, situated in the territory between Kilmacthomas and Bunmahon, is mentioned by O'Heerin and is still represented there. Several notable Keanes came from Munster, such as August Henry Keane (1833-1912), the anthropologist; John Lord Keane (1781-1844), a soldier of renown. Charles Kean (1811-1880), the actor, was born in Waterford, and his father, Edmund Kean (1787-1833), a still more famous actor, is stated by O'Donovan to be of the O'Cein sept.