Recently, a genealogist with a brick wall at an emigrant Irish ancestor born in the 1840s posted to a group seeking advice on getting documentation to reveal his specific origins. As usually happens, the ancestor had a very common Irish name and nothing in US records was particularly specific about where he came from. Given the general scarcity of documentation capable of resolving these situations, suggestions quickly turned to autosomal DNA testing. The original poster indicated she had heard of DNA testing, but was not sure about doing it herself.
Such reticence can eliminate, forever, the chance of breaking through brick-walls at this typical Irish emigrant timescale.
Realistically, autosomal DNA testing is the best tool for connecting an Irish immigrant to his or her origin family and townland back in Ireland, given the preponderance of common names in Ireland and the typical lack of specific birth locations on later-life records in many destination countries. And, for it to work, we really need to test the great-great grandchildren of those who were born in Ireland in the period just before and after the famine. Some of these 2x great-grandchildren are still with us.
There are technical and practical reasons for this.
For diaspora researchers, once census, marriage, death, and obituary record leads have dried up, what can work -- and work very well -- is discovering a second or third cousin match at one of the DNA testing companies who happens to have more information than you do.
Someone who matched at predicted third-cousin level to an Irish-descended elder in my family knew she had a line leading back to a Mary (with a very common Irish surname) who married a Patrick (with an even more common surname) both of Ireland, whose daughter came to NY. Mary and Patrick were born in the 1840s. Mary's surname matched with an Irish line that we know a lot about -- one we are still in contact with back in the Old Country. Our family therefore has an unusual amount of knowledge well beyond what is recorded in expected documentation. We've also gone through parish records from the 1820s forward and plucked out the baptisms and marriages that we know from locations, dates, witnesses, and naming patterns belong to our family. Where possible, we have down-traced using records combined with the memories of elders about first and second cousins they met in passing as children in the 1930s, further filling in this tree.
Mary's name and surname, once given to me by the match, was of obvious immediate interest, and I checked my database and discovered we did indeed have a 2nd great aunt who had married a Patrick with the given surname. The resulting relationship was consistent with the DNA shared and it was clear we had figured out the Most Recent Common Ancestor within minutes. The match got to add three more generations and many cousins to her tree. Mary and Patrick had married and moved away, and their children had gone even further, to upstate NY and then California, and so for us, this was really the only way to gain knowledge of what happened to her descendants.
Now, we were particularly lucky to have the two names of a couple to work with, and that Mary had married in her home parish (as expected) so that her husband's name was already in my database. While it would have been trickier, it still would have worked if we had only her name to go on. However, what likely would not have worked was if we waited to test the grandchild of either of the actual DNA testers. This is why:
Given that the probability the DNA tests can detect a cousin relationship drops off precipitously at the fourth cousin-level, waiting another generation or two is disastrous for solving these kinds of problems.
Additionally, looking at the estimated birth years in the above graphic, and taking into account the general history of Irish emigration (heavy starting in the mid 1800s) it is particularly important to test Irish diaspora descendants born in the 1920s and 1930s while we still can.
Once the elder generation has tested, it can take time for the autosomal DNA testing to bear fruit. For the first five years or so, I found it very slow going. However, so many more people are testing now, and some of them have good information, that we are generating results much more frequently and quickly. Once one Most Recent Common Ancestral couple is determined, that information becomes useful in figuring out potential relationships with other people who share that segment. And when people post the best trees that they can with their DNA testing profile, the linking can go like lightning.
The frustrating ambiguities inherent in researching immigrant ancestors who came from "Ireland" and often shared their name and birth year with many thousands of their countrymen persists. However, if one branch of the family happens to have knowledge going back 200 years and tests their descendants they can then help likely matches from other branches figure out how they fit in.
Large Irish families actually increase the odds of being able to sort it out (eventually) because all you need is for one branch to have remained relatively well established and documented. While all the other siblings may have dispersed around the world and to locales with next-to-useless documentation, if you can still test people in the needed senior generations, the match will show up and hopefully the pieces will then fit together. We found that good documentation happened with the eldest male branch which remained in Ireland, and then with the extended NY family that established itself in an around Brooklyn and still holds family reunions.
If we can piece together the family groups that constituted the population of Ireland throughout the nineteenth century, we may then be able to take it back even further. If we do not test Irish descended elders born in the 1920s and 1930s while we still can, this possibility will evaporate.