I just returned from the huge Dublin Irish Festival outside Columbus, Ohio: www.dublinirishfestival.org where I was able to consult with many patrons in the Genealogy Tent. One question which kept coming across my table was about the “O” and the “Mc” in Irish and Scots-Irish surnames.
“Mc” or “Mac” means “son of” among the Irish and the Scots. In Ireland into the Middle Ages naming was patronymic, with the next generation having a different surname depending on what the father’s first name was. The practice lasted longer in Scotland. It was by the eleventh century that the surnames became established in Ireland and did not change.
Now concerning the Irish “O” this meant “grandson,” and was used in a similar fashion as discussed above. This became standardized in the Middle Ages as surnames. A good discussion of this can be found on the “Irish Times” website by genealogist John Grenham.
Now concerning the use of “O” and “Mc/Mac” remember that what the ancient Gaelic folk used, and what we see today when Anglicized may be two different things. O’Murphy became Murphy, which is a simple example. To follow with this example, the records you use in genealogy will list Murphy far more often than O’Murphy.
Now for your research, there are some things you need to be aware. The name O’Connor and Connor are the same. Keep the “O” or drop the “O” (or Mc in other names) doesn’t make any difference. The ancestors were fluid in their thinking. My rule of thumb is to think phonetically; considering the majority of the population was either illiterate or semi-literate.
If you try and think phonetically, with or without the “Mc” or “O” then you will most likely find your people in the records where otherwise you may miss them. Also, when looking at various indexes, see if the compiler made a special section for “O” and “Mc” names. This is very important in your research.