In the course of this website I have had some emails from those involved in surname groups questioning the accuracy of some maps. I have used 2 sources in making these maps: (1) 1911 census of Ireland (2) MacLysaghts book: 'The Surnames of Ireland.' As a scientist I am trained to look for, or spot paterns emerging in the results. With surnames that pattern can reflect ethnicity. For example Irish Surnames (lets say those of Gaelic Irish origin) typically begin with O' or Mac, people with these 'Irish Surnames' are also typically over 90% Catholic in 1911. So heres the criteria I used:

1. Gaelic Irish Surnames: If MacLysaght indicates that one may be so, then it is listed as such as long as there is an overwhelming Catholic religious affiliation  in 1911. The Mc' or O' may be missing but that was a trend evident for the preceding two centuries. Many have also been anglicised (e.g. McGowan becomes Smith).
2. Viking Surnames: If there is a wiff of Viking ancestry then this is alluded to in the images, for example if the surname like O'Rourke and McLaughlin is derived from a Viking personal name. For some Viking surnames one does see it clustering to areas where there are Viking towns: Wexford, Waterford, Cork, and Limerick (plus of course Dublin).
3. Norman Surnames: Norman surnames show a typical pattern that spreads out from the southeast (Wexford). Also the religious affiliation of Norman surnames is predominantly Catholic (but often below 90% in 1911). 
4. Gallowglass Surnames: The mercenary Scots came and went over a 400 year time period but some remained or at least left their DNA signature with the locals. I have taken a somewhat strict criteria here; Any surname that can be of Irish or Scottish origin but with a Catholic religious affiliation greater than that typically observed with 'Planter Scot' surnames  (see below) is classed as potentially Gallowglass.
5. Plantation Scots/English Surnames:  It is a hisorical fact that these 16th and 17th century settlers did not mix very well with the locals. They were Protestant while the locals were Catholic, they were English speakers and came from the Scots and English border area (they were mainly lowlander Scots compared to the Gallowglass from the Highlands, Islands and Galloway). As such Scots and English Surnames that arrived in the 16th and 17th Century are overwhelingly Protestant in 1911 (greater than 90%). 
6. Surname Variants: What I have found is that as one moves away from a County of origin the spelling of the Surname often changes. For example my surname Bowe, found in Laois became Bowes in Dublin. What happens is simply that an administrator unfamiliar with a surname from a neighbouring county spells the surname as he hears it and as such it changes the further one moves away from the county of origin.

This is the criteria that I set out. It is not ideal as there are plenty of 'outliers' that do not fit the mould. Please remember that there are many maps and someone is bound to be unhappy with a missing O' or Mac' it is no cause for offense. If you are finding these maps upsetting you are obviously missing the 'message' from the website; 'in that the surnames of one's genetic matches can pinpoint where your ancestor lived when surnames became common over 1,000 years ago.' The surname maps are there to guide you to that county of origin, what I like to call your 'Genetic Homeland.'