This is often forgotten or confused by modern genealogists. For example there were two different O'Connor clans in Munster. O'Connor of Corcomroe was a fairly minor but distinguished family in west Co. Clare, or the Gaelic lordship of Thomond, as the area was known in early modern times. O'Connor Kerry was a very prominent family situtated in the north of that county. Before the arrival of the Anglo-Norman Fitzgeralds in north Kerry, the O'Connors had ruled the entire kingdom of Ciarraige Luachra. When the Fitzgeralds conquered the district the O'Connors of Kerry were confined to the marshy north of their territory along the banks of the river Shannon, where however, they prospered and had a famous castle. In the province of Leinster the ruling familiy of the Uí Failge people took the surname O'Connor and were known in early modern times as O'Connor Faly. This very distinguished family ruled the eastern half of modern Co. Offaly (which is called after them). The family became powerful supporters of the famous Fitzgerald earls of Kildare. When the English king, Henry VIII destroyed the House of Kildare in the 1530s, the O'Connors of Offaly went into rebellion and were a major thorn in the side of the English administration in Dublin for many years. As a result in the 1550s the English invaded Offaly and conquered the O'Connors, establishing the first plantation of English soldiers and settlers on the O'Connors' confiscated territory.

There was also a small O'Connor clan in the medieval Cenél nEógain kingdom of Tyrone in what is now Co. Derry. The head of this family was known as 'O'Connor, Lord of Cianachta Glinne Geimhin'. This family are very unusual however, in that they were not of the Cenél nEógain, but were descended from Tadc mac Céin, the grandson of an important figure from Irish mythology, Ailill Ólum. As such the O'Connors of Derry were related to such families as O'Hara and O'Gara in Connacht and O'Carroll of Ely in Munster. O'Connor lords of Gleann Geimhin are mentioned in the Annals of the Four Masters in the years 1094 - 1104, after which they disappear from the records like so many other medieval Tyrone families. However, when he visited Co. Derry for the Ordnance Survey in the mid nineteenth century, the famous Irish historian, John O'Donovan, found the O'Connors still living and farming in the parish of Dromachose.

However, it is with the province of Connacht that the O'Connors are rightly most associated with. In medieval times the O'Connors were kings of the province and even on occasion the most powerful O'Connor kings were High-Kings of Ireland. The O'Connors were the leading family of the Uí Briúin Aí dynasty, which was situated in central Connacht. Other branches of the Uí Briúin dynasty adopted other well known surnames such as O'Rourke (Uí Briúin Bréifne), and O'Flaherty (Ui Briúin Seóla). There were many powerful early O'Connor kings of Connacht such as Hugh of the Broken Spear (Áed in Gái Bernaig), who died in the year 1067 AD, but the first of the family to make a big impact on Irish history was the very powerful and resourceful King Turlough O'Connor, who died in 1156. Turlough's son Rory O'Connor became Ireland's last High-King in the few short years between 1166 and the arrival of of the first large contingents of Anglo-Normans in 1170-71. In the year 1171 King Rory O'Connor gathered a very large army to besiege and force the Anglo-Normans out of the city of Dublin, which they had captured the year before. However, as the siege dragged on, one day the Norman knights 'suddenly issued forth from the city about an hour after noon'. They immediately began to attack the surprised Irish warriors. One knight 'transfixed two of them with his lance'. Rory O'Connor, the high-king, was ignominiously surprised in his bath. He fled, leaving behind him not only his clothes but also the high-kingship of Ireland.

Rory O'Connor and his sons managed to hold onto the kingdom of Connacht and keep the Anglo-Normans out. Rory himself died in the year 1198 and was buried in the monastic town of Clonmacnoise. The Gaelic Irish appear to have had a great deal of respect for him as the last High-King of Ireland. In the year 1207 his body was exhumed 'and deposited in a stone shrine'. Rory's brother, Cathal Crobderg O'Connor, was probably the last old style Gaelic king in Ireland. Cathal Crobderg became king of Connacht in 1202, and until his death in 1224, he too kept the Anglo-Normans from crossing the Shannon. During his reign the Gaelic annalists referred to the Kingdom of Connacht as 'the best province in Ireland at that time'. Unfortunately for the O'Connors the kingdom of Connacht fell apart after the death of Cathal Crobderg O'Connor, and the Anglo-Normans finally invaded and conquered the most fertile parts of the province in the years after 1230.

There continued to be O'Connor kings of Connacht until the 1360s, but they ruled an ever decreasing area centred on what is now central Co. Roscommon. Eventually the family split into lots of minor branches such as O'Connor Don and O'Connor Roe in Roscommon, and O'Connor Sligo to the north in Lower Connacht (now called Co. Sligo after them). There were also wandering branches of the family such as the Clann Murtough O'Connors, who are recorded in the annals from the 1230s to the 1420s but eventually died out. Most of these O'Connor families were very weak politically and militarily in early modern times, although, from time to time, O'Connor Sligo chieftains were important figures in Gaelic Ireland.

So this is a very quick look at the many O'Connor clans of Ireland. At first look the proliferation of O'Connor families may appear very confusing to modern genealogists. However, if the records of any O'Connor are carefully assessed it should be very easy to determine which different family they belong to. A wealth of information is then available to the genealogist - the Irish annals are full of references to all these famous O'Connor families.