It's All in the Name
There are a number of family names that are widely known to be Irish. Some of them are very old indeed, dating back to the Celtic peoples who controlled the island many centuries ago. Other names were added later, as waves of invaders gradually merged with the Irish population. Thus, Irish surnames include Old Gaelic (O'Brien, O'Neill), Norman (FitzGerald, Butler), Scots (Hamilton), as well as a plethora of English names (White, Williamson).
About 10 percent of Irish surnames come from the Anglo-Normans of the twelfth and thirteenth centuries — the French, English, Welsh, and Flemish. A large proportion of surnames in the North come from Scotland, due to the Ulster Plantation in the 1600s. Furthermore, the English government Anglicized many old Irish surnames.
The Irish didn't really use family names until the tenth century, when Brian Boru supposedly introduced the practice. Instead, they identified themselves as the son or descendant of a father or earlier ancestor. This practice was continued in the many O's and Mcs that precede Irish names.
