Family Tree DNA: Genetic Testing Service
DNA testing will show your connections with other families and ethnic groups. The database includes many peoples from throughout Europe and the rest of the world including Irish, English, Scots, and members of many other ethnic groups. If you're Irish you can join their "Ireland Y-DNA Project" and/or the "Ireland Heritage mtDNA Project". They also have more specialized projects like "R-L21 South Irish" and "Munster Irish".
23andMe: Genetic Testing Service
You can test your autosomal DNA here and look for Irish and non-Irish matches in their extensive database. The associated project "Ireland XO" is developing a genetic database of patterns in particular Irish parishes.
Important research is also underway in the "Irish DNA Atlas" project run by the Genealogical Society of Ireland, the Royal College of Surgeons in Ireland, and the University of Leicester. They seek people whose 8 great-grandparents were all born in Irish towns within 30 kilometers of each other.
The "Celtic" Irish people of the emerald isle of Ireland are closely related to the Scottish people of nearby Scotland, and Irish and the partly Frisian-Anglo-Saxon English people from England are also significantly related. This shows the limitations of assuming we know everything about somebody's ancestry merely based on what language their ethnic group traditionally spoke (in this case, Irish Gaelic versus English). Also, some Irish people moved to Iceland and are thus partly related to modern Icelanders.
R1b, which originated in western Europe, is the most common Y-DNA haplogroup among Irish men, at a frequency of about 81.5%. I1 is the second most common with 6%, followed by I2b at 5%, R1a at 2.5%, and E1b1b at 2%. G2a is found in only about 1%. Also rare are I2a (1%) and J2 (1%).
In terms of maternal mitochondrial DNA (mtDNA), about 38.5% of Irish people carry mtDNA haplogroup H (of whom 11% are in H1 and H3), 13% carry U (of whom 2% are in U2, 0.5% are in U3, 2.5% are in U4, and 6% are in U5), 12% carry T, 11% carry K, and 10% carry J. Several others are encountered at smaller frequencies: 4% in HV0 and V, 3% in I, 2.5% in W, 1.5% in X2, and 4.5% in other(s).
Laoise T. Moore, Brian McEvoy, Eleanor Cape, Katharine Simms, and Daniel G. Bradley. "A Y-Chromosome Signature of Hegemony in Gaelic Ireland." American Journal of Human Genetics 78 (February 2006): pages 334-338. First published online. The researchers used 17-marker simple tandem repeat (STR) analysis on the Y chromosomes of samples obtained from Irish men. They discovered that 16.9% of men from northwestern Ireland, and 8.2% of men from Ireland as a whole, descend from a single male ancestor from early-medieval times from the family dynasty of the Uí Néill, since the haplotype is often found in people holding surnames associated with this dynasty. Their abstract calls this a "modal haplotype".
Colm T. O'Dushlaine, Derek Morris, Valentina Moskvina, George Kirov, International Schizophrenia Consortium, Michael Gill, Aiden Corvin, James F. Wilson, and Gianpiero L. Cavalleri. "Population structure and genome-wide patterns of variation in Ireland and Britain." European Journal of Human Genetics 18 (2010): pages 1248-1254. First published online on June 23, 2010. The researchers studied the genetics of 3,654 including people from Ireland, the United Kingdom (including Aberdeen, Scotland), Sweden, Portugal, Bulgaria, and the American state of Utah (whose people are largely of English descent). Haplotype diversity was found to be lower in Ireland and Scotland than in southern Europe. Also, Irish people have higher levels of linkage disequilibrium and homozygosity compared to other Europeans. The results showed that the population of Ireland has been relatively isolated throughout the millennia. The article notes that Scottish people are "intermediate between the Irish and English cohorts" in principal component analysis. British and Irish people are predominantly "Northwestern" European in origin but also partly "Scandinavian" (moreso for English people than Irish people) and have relatively small amounts of "Iberian" and "Balkan" ancestry.
Admixture results based on the Dodecad Ancestry Project showed, at K=11, that Irish are mostly "Northwest European" (as we'd expect), also partly "Northeast European" and "Basque", with a small slice of "Sardinian", and a little bit of "West Asian". The Irish are very similar to British, which is also shown by their clustering together in two main groups.
Brian McEvoy, Claire Brady, Laoise T. Moore, and Daniel G. Bradley. "The scale and nature of Viking settlement in Ireland from Y-chromosome admixture analysis." European Journal of Human Genetics 14 (2006): pages 1288-1294. First published online on September 6, 2006. The researchers studied the Y-DNA of Irish men with surnames considered to be of Norse origin. They examined both unique event polymorphisms and short tandem repeat (STR) markers. They found that these Irish men actually didn't usually have paternal roots from Scandinavia, nor do Irish men in the general population of modern Ireland. Excerpts from the Abstract:
"[...] the findings are consistent with a relatively small number of Norse settlers (and descendents) migrating to Ireland during the Viking period (ca. AD 800-1200) suggesting that Norse colonial settlements might have been largely composed of indigenous Irish. [...]"
John H. Relethford and Michael H. Crawford. "Genetic drift and the population history of the Irish travellers." American Journal of Physical Anthropology 150:2 (February 2013): pages 184-189. First published online on November 26, 2012. Found that members of the Irish Travellers nomadic subculture of Ireland are of ethnic Irish descent and not genetically related to the Roma people or modern-day people of India. Genetic differences between Irish Travellers and other Irish people are explained by genetic drift.
Valentina Moskvina, Michael Smith, Dobril Ivanov, Douglas Blackwood, David StClair, Christina Hultman, Draga Toncheva, Michael Gill, Aiden Corvin, Colm O'Dushlaine, Derek W. Morris, Naomi R. Wray, Patrick Sullivan, Carlos Pato, Michele T. Pato, Pamela Sklar, Shaun Purcell, Peter Holmans, Michael C. O'Donovan, Michael J. Owen, George Kirov, and the International Schizophrenia Consortium. "Genetic Differences between Five European Populations." Human Heredity 70:2 (July 2010): pages 141-149. The researchers tested Irish, Scottish, Swedish, Bulgarian, and Portuguese people to study the differences in their SNP allele frequencies, based on 40,593 SNPs. Not surprisingly, Irish and Scottish people were found to be relatively closely related because the differences between their SNPs were the lowest found between any two peoples in the study. Excerpts from the Abstract:
[...] The largest differences clustered in gene ontology categories for immunity and pigmentation. Some of the top loci span genes that have already been reported as highly stratified: genes for hair color and pigmentation (HERC2, EXOC2, IRF4), the LCT gene, genes involved in NAD metabolism, and in immunity (HLA and the Toll-like receptor genes TLR10, TLR1, TLR6). However, several genes have not previously been reported as stratified within European populations, indicating that they might also have provided selective advantages: several zinc finger genes, two genes involved in glutathione synthesis or function, and most intriguingly, FOXP2, implicated in speech development. [...]"
Sophie I. Candille, Devin M. Absher, Sandra Beleza, Marc Bauchet, Brian McEvoy, Nanibaa' A. Garrison, Jun Z. Li, Richard M. Myers, Gregory S. Barsh, Hua Tang, and Mark D. Shriver. "Genome-Wide Association Studies of Quantitatively Measured Skin, Hair, and Eye Pigmentation in Four European Populations." PLoS ONE 7(10) (October 31, 2012): e48294. Published electronically. As expected, Irish people were found to have overall lighter skin pigmentation than continental Europeans. The article also confirmed that, on average, the hair colors of Irish and Polish people (northern Europeans) are lighter than Italian and Portuguese people (southern Europeans). Within Ireland, Irish females have a pronounced tendency toward lighter hair than Irish males; a sexual dimorphism of this magnitude wasn't detected in the Poles. Northern Europeans also have, on average, lighter eyes than southern Europeans. Furthermore, both Irish males and females tend to have lighter eyes than even Poles do. Excerpts from the Abstract:
"Pigmentation of the skin, hair, and eyes varies both within and between human populations. [...] Objective and quantitative measures of skin, hair, and eye color were made using reflectance or digital spectroscopy in Europeans from Ireland, Poland, Italy, and Portugal. A GWAS was conducted for the three quantitative pigmentation phenotypes in 176 women across 313,763 SNP loci, and replication of the most significant associations was attempted in a sample of 294 European men and women from the same countries. We find that the pigmentation phenotypes are highly stratified along axes of European genetic differentiation. The country of sampling explains approximately 35% of the variation in skin pigmentation, 31% of the variation in hair pigmentation, and 40% of the variation in eye pigmentation. All three quantitative phenotypes are correlated with each other. In our two-stage association study, we reproduce the association of rs1667394 at the OCA2/HERC2 locus with eye color but we do not identify new genetic determinants of skin and hair pigmentation supporting the lack of major genes affecting skin and hair color variation within Europe and suggesting that not only careful phenotyping but also larger cohorts are required to understand the genetic architecture of these complex quantitative traits. Interestingly, we also see that in each of these four populations, men are more lightly pigmented in the unexposed skin of the inner arm than women, [...]"
John Holden. "Is distinctive DNA marker proof of ancient genocide?" Irish Times (June 13, 2013). Alastair Moffat of IrelandsDNA believes that the high frequency of Y-DNA haplogroup R1b compared to G in Irish men suggests an invasion of R1b carriers that overwhelmed carriers of the Y-DNA haplogroup G. Moffat thinks the newcomers came from lands to the south, such as France or northern Spain.
Damian Corless. "DNA blueprint of the Irish revealed." Irish Independent (September 11, 2010). This article is based on research by Brendan Loftus of University College Dublin, whose "research team [...] mapped the complete genetic code of an Irish person for the first time." Researchers hope that analysis of the Irish genome will help to explain why Irish people are susceptible to particular disorders and try to find preventative measures and cures for those disorders. Excerpts from the article:
"[...] Ireland's geography has had a huge part to play in shaping the nature of our society and our closest family ties. According to Loftus: 'The geographic isolation of Ireland over generations would affect the size of the gene pool by limiting the type and number of potential mating partners.' Major genetic surveys of Ireland and Britain have established that the gene pool of both islands is amongst the least diluted in Europe. The genetic evidence shows that three quarters of the ancestors of the Irish and British people were the pioneering settlers who arrived at the end of the last ice age between 17,000 and 8,000 years ago. The inescapable upshot of this is that the Irish are not Celts, any more than the English are Anglo-Saxons. In fact, both the Irish and the British are Basques, with the Irish significantly more Basque than our neighbours across the pond, who've absorbed more migrations from Europe over the centuries. Scientists estimate that Ireland's gene pool has changed remarkably little since the first hunter-gatherers from Iberia followed the retreating ice cap, beachcombing northwards and settling this newly exposed and empty land. The dilution rate for Ireland is estimated at a tiny 12%, against 20% for Wales and Cornwall, 30% for Scotland and 33% for England. [...] Ancient Irish legends say that there were six invasions or migrations from the south many generations before the Celts arrived around 300BC. The evidence suggests that the Celtic language, fashions and technologies which are supposed to define our Irish heritage, were acquired as cultural accessories [...] The Irish and Basques share by far the highest incidence of the R1b gene in Europe, which has a frequency of over 90% in Basque country and almost 100% along parts of Ireland's western seaboard. If further proof were needed, there's the physical fact that the Basques are distinguished by a very high incidence of fair (and some reddish) hair, pale skin, blue eyes, and, apparently, sticky-out ears. Sound like anyone you know? [...]"
Nicholas Wade. "English, Irish, Scots: They're All One, Genes Suggest." The New York Times (March 5, 2007). Geneticist Stephen Oppenheimer of the University of Oxford used genetic evidence to disprove the traditional historical narrative that the Irish people are mainly Celts and that they're very distinct from Englishmen. Oppenheimer suggested, rather, that most of the ancestors of Irish, Scottish, Welsh, and English people came from Spain about 16,000 years ago and that their original language was related to Basque. Excerpts:
"[...] Agriculture may have been introduced by people speaking Celtic, in Dr. Oppenheimer's view. Although the Celtic immigrants may have been few in number, they spread their farming techniques and their language throughout Ireland and the western coast of Britain. Later immigrants arrived from northern Europe had more influence on the eastern and southern coasts. They too spread their language, a branch of German, but these invaders' numbers were also small compared with the local population. In all, about three-quarters of the ancestors of today's British and Irish populations arrived between 15,000 and 7,500 years ago, when rising sea levels finally divided Britain and Ireland from the Continent and from one another, Dr. Oppenheimer calculates in a new book, 'The Origins of the British: A Genetic Detective Story' (Carroll & Graf, 2006). As for subsequent invaders, Ireland received the fewest; the invaders' DNA makes up about 12 percent of the Irish gene pool, Dr. Oppenheimer estimates, but it accounts for 20 percent of the gene pool in Wales, 30 percent in Scotland, and about one-third in eastern and southern England. [...]"