Some of the hereditary Gaelic surnames took form at periods as early as the ninth and tenth centuries. These names are the earliest substantiated European records of family names. The majority however, like the surnames met in France and Italy, appear to have been adopted in the eleventh or twelfth centuries as in the case of the family name O'Dalaigh.
The earliest recording of the name was Cuchonnacht O'Dalaigh, who lived in Teffia, in what is now the County of Westmeath. Being noted for his learning, he was called "Cuchonnacht na Sgoile," meaning "Cuchonnacht of the School." He died in the year 1139.
The "O" prefixed to the name "Dalach" or "Dalaigh" signifies grandson or male descendant of Dalach in contrast to the prefix Mac or Mc which would mean son of the ancestor specified.
The modern Anglicised versions of the ancient Gaelic family name Ua Dalaigh or O'Dalaigh, pronounced "O'Dhaulee," came about as a result of tyrannical laws, designed by the English rulers, which outlawed Gaelic names and customs as a means of penalizing the Irish people into a state of subjugation. Beginning with the reign of the English King Edward IV (1465), the law demanded that every Irishman living within the territory known as the "Pale" take an English name and comply with other English customs or forfeit his possessions.
Most families resisted and it wasn't until the close of the Seventeenth Century, after constant persecution and ridicule that the Gaelic forms nearly dissapeared. In many cases the Anglicised forms were actually closely related to the basic Gaelic surnames minus the "Macs" or the "O's" as in the case of the name Daly.
The final doom of the Gaelic surnames came as a result of the widespread establishment of the English language among the Irish. This circumstance caused the English form of names to be taken for granted and considered as natural.
In recent times, the Gaelic original has been re-adopted by some of the families.
This trend back to the Gaelic will naturally spread as the ancient language gradually resumes its old dominant position in Ireland.