And that "but" represents the fact that different Irish surnames arose at different times, usually between 900 to 1350 AD, and mostly between 950-1150 AD, with the busiest period being 1000-1050 AD. Also, surnames with the prefix O or Ó were formed prior to 1200 AD and those that formed afterwards were mainly those with the prefix Mac. These are the top-line conclusions from the data analysis that follows.
Back in 1923, Woulfe claimed that "Irish surnames came into use gradually from about the middle of the 10th to the end of the 13th century".  But let's look at some of the hard evidence supporting this statement. I have gathered such data from two sources - the introduction to the 1923 edition of Woulfe's Irish Names and Surnames  and a more recent 1999 journal article by Ó Murchada. 
In his 1999 review article, Ó Murchada provides a table in the appendix with years of death for 78 progenitors of a selection of Irish surnames.  The complete list of 78 surnames is included in Footnote 1.
Ó Murchada suggests that the average surname would have come into use during the time of the progenitors' great-grandsons.
Using Corpus Genealogiarum Hiberniae as source, I extracted thirty genealogies which could be traced in the annals and dated for at least ten generations. This gave a total of 416 generations, which when divided into a total of 13,779 years, furnished an average generation gap of 33.2 years, i.e. the number of years between the death of a father and that of his son / successor ... By my calculations, adding sixty-six years to the date of death of the eponym will give an approximate date for the death of his grandson, and at any time subsequent to that, in his great-grandson’s era, one could expect the surname to have come into use ... (Ó Murchada p30)
The earliest surname (O Clery) is probably the first fixed surname to be used in Europe. The originator died in 858 AD and the first record of its use is 58 years later in 916 AD in the Annals of the Four Masters. A second occurrence of this fixed surname occurs 34 years later in 950 AD. 
Woulfe gives other examples of specific dates when several fixed surnames were first mentioned in the ancient texts, illustrating that inherited fixed patronymic surnames were well-established before the turn of the first millennium. 
- Ó Canannáin (O'Cannon) ... 941 (he "flourished" in 950 AD so this surname appears to have been introduced during the lifetime of the progenitor. In the ancient texts, "flourished" simply means lived, indicated by the Latin word floruit, or the abbreviation fl.)
- Ua Néill (O'Neill) ... 943 (the progenitor was slain in 919)
- Ua Ruairc (O'Rourke) of Breifney ... 952 (progenitor died in 893)
- Ua Ciardha (O'Keary) of Cairbre ... 952
- Mag Aongusa (Maguiness) ... 956
- Ó Maoldoraidh (O'Muldory) of Tirconnaill ... <999 (progenitor flourished in 870)
- Ó Dubhda (O'Dowd) of Tireragh ... <999 (progenitor flourished in 876)
- Ó Ceallaigh (O'Kelly) of Ui Maine ... <999 (progenitor flourished in 874)
This data also disproves the mistaken belief that surnames were introduced by Royal Decree during the reign of Brian Boru (1002-1014). In fact the O'Brien surname (which derived from Brian's name) did not become a fixed inherited surname until the time of his grandsons. 
Note that it was not uncommon for the same surname to arise in different places and hence the clan territory is frequently added after the surname as a qualifier. For example, we have the O'Donnell clan of Corca Bhaiscinn, another of the same name in Ui Maine, and a third in Tirconnell. And there was also the O'Conor clan of Connacht, another in Corcomruadh, and a third clan in Offaly.
So, adding up the various dates of surname introduction in Ó Murchada's article and dividing by 78 gives an average date of 1072 AD for the introduction of surnames in Ireland. As we can see in the bar chart below, the majority of the surnames were introduced between 950 and 1150 AD (59/78 = 76%), with the time period 1000-1049 being the busiest for surname introduction.
|Estimated dates of surname introduction for 78 Irish surnames - data extracted from O'Murchada 1999 |
A similar pattern is seen with the data extracted from Woulfe.  In the Introduction to the hard copy of his Irish Names and Surnames (page xvi), Woulfe lists 46 surnames that had previously been compiled by O'Donovan and extracted mainly from from the Annals of Ulster and the Annals of the Four Masters. The list (see Footnote 2 below) includes dates when the various progenitors flourished, died or were slain.
Woulfe states that the date when the surname became fixed "cannot have been more than 60 years from the period when the ancestor flourished or died". However, in an attempt to standardise the dates, I have added an additional 10 years to any dates when they "flourished" (to give an approximate date of death or manslaughter) and then added 60 years on top of this. This gives an average of 66 years between the death of the progenitor and the introduction of the surname ... which is the same interval employed by Ó Murchada above.
Based on these adjustments, the average year for the introduction of these 46 surnames was 1048 AD. The earliest was 920 AD and the latest was 1350 AD. The largest number of surnames appeared in the period between 1000-1049 AD with 83% (38/46) of surnames being introduced between 950-1150 AD.
|Estimated dates of surname introduction for 46 Irish surnames - data extracted from Woulfe 1923 |
(based in turn on O'Donovan)
Combining the two datasets and removing any duplicates (or likely duplicates) produced 109 distinct surnames (see Excel spreadsheet here). The average year of introduction of surnames was 1068 AD (range 920-1350) but the busiest period for surname introduction was again 1000-1049 AD. The majority of surnames (84/109= 77%) were introduced in the period between 950-1150 AD.
The proportion of surnames introduced in successive centuries is as follows:
- 10th Century ... 24/109 = 22%
- 11th Century ... 50/109 = 46%
- 12th Century ... 26/109 = 24%
- 13th Century ... 7/109 = 6%
- 14th Century ... 2/109 = 2%
|Estimated dates of surname introduction for 109 Irish surnames - combined dataset|
In the combined dataset, surnames prefixed with O or Ó arose between 920 and 1193 AD, and surnames prefixed with Mac between 955 and 1350 AD (as illustrated in the bar chart below). The prefix Mac means "son of" and O or Ó means "grandson of" or alternatively "descendant of".
Interestingly, O surnames were introduced about 124 years earlier than Mac surnames (1032 vs 1155 AD for average dates of introduction). Woulfe states that the creation of surnames with the O or Ó prefix "had almost certainly ceased" prior to the Norman Invasion, and that surnames that arose thereafter were primarily of the Mac variety. This is almost true - in the combined dataset, only two O surnames arose after 1169 AD (the start of the Norman Conquest); one in 1170 (O'Shaughnessy) and another in 1193 (O'Growney) - see Excel spreadsheet here.
|Estimated dates of emergence of O and Mac surnames - combined dataset|
This data paints a very clear picture of the emergence of surnames in Ireland.
But from a genetic genealogy perspective, why is this important? Knowing when a particular Irish surname was first introduced will be of particular use to Surname Project Administrators as it identifies a maximum age for the genetic group that is presumed to be descended from that particular surname founder. This upper age limit can help constrain the date calculations for the various branches in this portion of the Tree of Mankind, including those branches below the overarching SNP for the surname in question, as well as the adjacent branches of different but genetically-related surnames.
However, the data presented above does not answer the questions: when did surnames in Ireland become commonplace? when were they adopted by the majority of the population?
And that is a topic for a subsequent article.
And that is a topic for a subsequent article.
 Woulfe, Patrick. Sloinnte Gaedheal is Gall: Irish Names and Surnames (1923 Dublin), pages xv-xx. Online version available at http://www.libraryireland.com/names/of/o-fearghail.php
 The Formation of Gaelic Surnames in Ireland: Choosing the Eponyms by Diarmuid Ó Murchadha, Locus Project, University College, Cork. Nomina (1999) - available at http://www.snsbi.org.uk/Nomina_articles/Nomina_22_OMurchadha.pdf
1) 78 surnames from Ó Murchadha  above ...