Well, somewhere around 10% of white people have a deficiency in some enzyme (CY-P450, I'm pretty sure) which affects people's reactions to various drugs. I remember reading this 10% figure in some book dealing with MDMA (PIHKAL perhaps, or Nick Saunders' book) which claimed that this deficiency made MDMA hit these people harder than usual. Apparantly this same enzyme deficiency also makes codeine less effective (they have a hard time metabolizing it into morphine, so it tends to make them nauseous more than narcotized). It also affects how DXM works (not surprising, since DXM is an opioid and this enzyme seems to play a big role in opioid metabolism). Grapefruit juice also acts to inhibit this enzyme.
Its an enzyme that affects MANY drugs, and different levels of this and related enzymes (theres a whole bunch of them, I think they're referred to collectively as cytochrome somethingorother) can have a drastic impact on how many drugs work, from MDMA to opioids to various heart medications. Its certainly possible that this enzyme plays a role either in 5-MeO-DIPT's metabolism or perhaps somewhere in the chain of physiological reactions started by the drug. We know surprisingly little about how psychedelics in general work, and practically nothing about the pharmacodynamics of 5-MeO-DIPT specifically.
As far as I know though, this particular enzyme deficiency isn't limited to Irish or other Keltic peoples... rather, the 10% figure is I believe something common to all white Europeans. Also the deficiency also occurs in non-whites, its just that its more common in people with white ancestry. I don't know if its a dominant or recessive trait and how it would affect people of mixed ancestry. I also don't know how accurate that 10% number is... the book was a book dealing with psychedelics, not genetics or enzymatic metabolism. I'm pretty sure that CY-P450 deficiency is something that occurs more frequently with whites than other groups but I'm not sure if 10% is really an accurate statistic.
Also, I'm not sure on the racial scope... Assuming the 10% figure is correct, does this apply to just white Europeans, or does it apply to Aryan people as a whole? And by this, I am using Aryan in its proper sense and not the misuse of the term by the Nazis... the proper definition of Aryan refers to people descended from a group of people who probably originated somewhere around the Black Sea or the Caucasus Mountains. These people then spread out in several directions in different migratory waves. We can tell where their descendants ended up because they all speak related languages, known collectively as Indo-European. The term Aryan properly includes most white Europeans (Kelts, Germans, Slavs, Latins, Greeks) but also includes many of the people of India, Pakistan, Iran, Afghanistan, and some of the people of the central Asian steppes. Is the enzyme deficiency as common with them? Dunno. Then, there's also white Europeans who pre-dated the Aryans... for example, the Basques, and probably the Etruscans and Minoans... and there are also later arrivals such as the Finns and Estonians, who are actually related to the Asiatic people of Siberia, and more distantly (based on some linguistic evidence) to the Koreans and Japanese. And of course, theres been so much interbreeding over the centuries, there's probably no people anywhere in the world who are genetically like the original Aryans. So, what does all this mean for the genetics of this enzyme deficiency? Hell if I know. Did it originate with the Aryans, and if so, does it occur as often in non-white descendants of the Aryans such as the people of India? Maybe it was something picked up through intermarriage with the pre-Aryan peoples of Europe, and is therefore found in all white Europeans whether of Aryan or pre-Aryan origins... or maybe it was a genetic mutation that occured in the Aryans who migrated to Europe and so doesn't affect Basques, Finns, etc. You'd want to ask a geneticist if you really care about all this crap.
Its also possible that there is some other unknown enzyme or receptor condition or god only knows what that has nothing to do with CY-P450 but plays a role in the pharmacology of 5-MeO-DIPT. If so this could be something found in Irish or other Keltic people. It wouldn't be that unusual. In addition to the CY-P450 deficiency common with whites, many Native Americans are deficient in the enzyme alcohol dehydrogenase which turns alcohols into aldehydes. This makes these people unusually sensitive to alcohol and prone to alcoholism - though of course the genocidal conditions on Indian Reservations probably contributes more to the epidemic of alcoholism than any genetic factors! There are other various odd reactions to drugs and foods that tend to be found mosty in specific races, so it would not be completely unreasonable to speculate that people of Irish descent may have some kind of enzyme or neuroreceptor mutation that makes them react to 5-MeO-DIPT differently.
Personally, I have lots of Irish ancestry, but I've also got ancestors from Germany, Holland, England and Indonesia, so that makes it hard to draw any conclusions from my experiences. I found low doses (4-8mg) to be a mild and somewhat interesting semi-psychedelic sensory enhancer that could perhaps have some recreational value for some people... if future research shows it to be a safe drug, and I came across some, I would consider taking it again at a dose around 5mg just for fun. I found that doses above 8mg did begin to create fully psychedelic experiences, but I found it pretty unusable in that capacity because along with the increase in psychedelic qualities came an increase in body load - mostly, intestinal upset. It seems to have an effect opposite of opioids in that it seems to speed up intestinal movements, causing gas and often severe diarrhea that can last hours. This didnt happen every time I did it, but it happened often enough and was unpleasant enough that I would never take this substance again at a dose above 8mg. In fact, I dont think I'd want to even go past 5mg. My idea of a good psychedelic experience does not involve spending 3 or 4 hours stuck on a toilet shitting out every last bit of liquid in my intestines.
Anyway, until we know everything about how 5-MeO-DIPT works and what chemicals in the body it reacts with, and can do tests on different pure-blooded representatives of different ethnic groups to determine their levels of those chemicals, this is something that we can only speculate about based on anecdotal evidence... and since there's probably not that many people using 5-MeO-DIPT who happen to be pure-blooded Irish, I don't think speculation is even possible. We don't have enough anecdotal evidence about human reaction to this drug in general to even think about how some populations may react differently from the norm. We don't know what the norm is, and probably won't for many many many years - if ever.