Inspired by tales which claim the darker features to be of Iberian derivation, knowledge seekers have looked to science for answers, often citing genetic studies pertaining to those with Irish (and/or British) ancestry. This is seen as a means of determining what genotypic and environmental factors have contributed to the divergence between the more or less prevalent types found among Irish people.
The term 'Black Irish' is also accompanied by claims suggesting these physical traits to be the result of an Iberian admixture originating with survivors of the Spanish Armada. However, the genetic contributions of the latter are likely to have been insignificant, as most Armada survivors were killed on the beaches, and many of the remnants eventually escaped from Ireland. It is believed that a group of Spanish soldiers ended up serving as armed retainers to the Irish chiefs Brian O'Rourke, Sorley Boy MacDonnell, and Hugh O'Neill, 2nd Earl of Tyrone. Consequently, these soldiers may have lived in Ireland long enough to father children, but did not constitute a very large number. The genetic evidence is that the survivors of the Spanish Armada probably left no legacy, as the Irish have only minute amounts of Neolithic Near Eastern Y chromosome genetic markers such as E3b and J, both of which are present in low, but significant, levels throughout Spain (with the exception of the Basque Country).
The Spanish Armada myth is thought to have been a corruption of a story based on the Milesians (not to be confused with the ancient Greek people of the same name), the purported descendants of Míl Espáine (Latin Miles Hispaniae, "Soldier of Hispania", later pseudo-Latinised as "Milesius"), speculated to represent Celtic-speaking peoples from the western Iberian peninsula who began to migrate to Ireland and Britain in the fifth century B.C.
Genetic research also shows a strong similarity between the Y chromosome haplotypes of males from northwestern Spain and Irish males with Gaelic surnames, with a sizeable difference between the west and the east of Ireland, in that much of those from the west owe less of their DNA to Anglo-Saxon and Scandinavian populations. Genetic marker R1b reaches frequencies as high as 98% in northwestern Ireland and 95% in southwestern Ireland, but drops to 73% in northeastern Ireland and 85% in southeastern Ireland. Additionally, R1b averages between 89% and 95% in Y chromosomes of the Basques of northern Spain (and southwestern France) considerably greater than levels of the same haplogroup found amongst the remaining Spanish genepool, where it varies from region to region in a range from 42% to 75%, but mostly with percentages in the 50s and 60s.
In recently published books (Blood of the Isles by Bryan Sykes and The Origins of the British - A Genetic Detective Story by Stephen Oppenheimer), both authors propose that ancient inhabitants of Ireland can be traced back to the Iberian Peninsula, as a result of a series of migrations that took place during the Mesolithic and to a lesser extent the Neolithic Age. These movements theoretically laid the foundations for present-day populations in the British Isles. According to journalist Nicholas Wade, Oppenheimer maintains there is a great lineal commonality between the Irish and British people, as reported in the March 6, 2007 edition of the New York Times. Oppenheimer also advances the controversial claim that a language closely related to Basque was long ago spoken by their shared ancestors.
In a statistical survey of the Irish carried out by Mr. C. Wesley Dupertuis in the 1940s under the endorsement and guidance of The Division of Anthropology of Harvard University, based on some 10,000 adult males, the following information was gathered and so documented.
The hair color of the Irish is predominantly brown. Less than 3% have black or ashen hair; 40% have dark brown hair. Medium brown hues make up another 35%. Persons with blond and light brown hair account for close to 15%, while approximately 10% have red hair. Both golden and dark brown shades can be seen in the southwestern counties of Ireland, but fairest hair in general is most common in the Central Plain. Ulster has been evidenced to have the highest frequencies of red hair with the lowest found in Wexford and Waterford.
In further examining pigmentation characteristics (both as a whole and regionally), studies have indicated the Irish are 'almost uniquely pale skinned when unexposed, untanned parts of the body, are observed' and '40% of the entire group are freckled to some extent.' Moreover, 'in the proportion of pure light eyes', data shows that 'Ireland competes successfully with the blondest regions of Scandinavia,' as approximately 42% of the Irish population have blue eyes. Another 30% have been found to possess light-mixed eyes and 'less than 1 half of 1% have pure brown.'
The complete results of this survey have been condensed and arranged in the Harvard Anthropometric Laboratory (formerly under the close supervision of Professor Earnest A. Hooton) with the cooperation of both governments in Ireland.
A lesser-known point of origin refers to the potato famine of 1845-1851, which turned the blighted potatoes 'black' and as a result drove thousands of Irish to America's shores.
The term has also been used to denote the offspring of Irish laborers and African slaves in the Caribbean. Montserrat, by far, experienced the highest concentration of Irish immigrants, as it was forcibly settled by the English crown using indentured servants from Ireland. These Irish servants were eventually replaced by West African slaves who took on the surnames of the prior inhabitants, much as African slaves in the United States assumed the names of their owners.
In the United States, whites with Native American, African American, or other non-white ancestry may historically have called themselves "Black Irish," "Black Dutch," or "Black German" as a reflection of their coloring.
A prominent theme of ethnology in Victorian England largely stemming from social prejudices of the time was that the Irish were racially different from the English people and thus considered inferior. Polygenism was a dominant theory, as was phrenology, and both were employed to 'prove' that Irish persons were less developed and more primitive than other 'races' of humanity. Punch cartoons often portrayed them with protruding jaws, alluding to the notion they were closer to apes than men.
John Beddoe (1826-1911), one of the most notable ethnologists in the United Kingdom, supported these concepts with his work. In The Races of Britain: A Contribution to the Anthropology of Western Europe (1862), Beddoe wrote that all geniuses were orthognathous (having the front of the skull, almost vertical, not receding above the jaws), as opposed to the Irish and Welsh whom he exaggeratedly described as prognathous. Evasive or ignorant of the pre-Saxon Celtic influence on the English and likely his own forebears, Beddoe claimed that the Celts were closely related to Cro-Magnon man, theorized by him, as being linked to the 'Africanoid'. The Races of Britain was republished in 1885, 1905, and again in 1971.
In the pilot to NBC's television series The Black Donnellys, Joey Ice Cream, narrator and one of the mainstay characters, indirectly refers to himself and unspecified Donnelly brothers as Black Irish. Joey cites a story told by his grandmother which asserted that Ireland was originally inhabited by a 'dark haired race of people' whom the invading Celts attacked, but failed to wipe out.
On the HBO series Oz, Ryan O'Reily describes himself as Black Irish, and convinces Sean Murphy, a corrections officer, to befriend him by pointing out that they are both Black Irish. Murphy responds that they are "as black as they come".
In an episode of the NBC series "30 Rock", Conan O'Brien, portraying himself, refers to Alec Baldwin's character, Jack Donaghy as a "Black Irish bastard," to which Donaghy replies, "Back at you, red."
The main character of Urban Shaman, Joanne Walker/Sioban Walkingstick, repeatedly refers to herself as 'Black Irish' due to her Cherokee and Irish parentage.
In Tennessee Williams's play "The Night of the Iguana" the main character T. Lawrence Shannon is described as Black Irish by the other characters.
In the musical "The Producers", one of the policemen that come to arrest Max Bialystock and Leo Bloom is African-American and speaks with an Irish accent. Upon hearing him speak, Max turns to the audience and says, "I've heard of black Irish, but this is ridiculous!"