What is my excuse?I don't have an excuse, I have a reason.I don't believe in wasting money.Neither, I think, does my BiL.Nor do I have an "offical position" to take up; I'm retired.
I also have half a working lifetime as a biological scientist.Most of that career was spent in a field which involved interpretation of genetic markers to help establish identity, albeit before DNA sequencing became the technique de jour.I also spent some time in palaeoecological research (or archaeobotanical if you prefer) in Ireland.
That experience has taught me to take a very conservative, critical, even minimalist view of evidence and to resist the siren call of the short cut.In this instance that view is that the only actual piece of factual information you have is that your other half carries a marker which is shared by about 1 in 5 males in NW Ireland.
If you read carefully the blurb you cut and pasted you will see that it says 'a striking % of men in Ireland (and quite a few in Scotland) share the same Y chromosome, suggesting that the 5th-century warlord known as "Niall of the Nine Hostages" **may** be the ancestor of one in 12 Irishmen'.
Note the key word "may".In other words it's a moot point.What is far less questionable is that a large proportion (and I'll consider how large a proportion later) of modern males must have lines of male descent from a common ancestor at some period.AIUI (and this is from a quick read of stuff found on the web, I haven't yet had time to seek out the Nature and Am J. Gen. papers) the TCD argument is that Niall was a powerful individual and this power enabled him to be the ancestor of a disproportionate part of the population and hence to be that common ancestor.
Unless the population was small Niall and his male descendants in the next few generations must have been extraordinarily active to have achieved this.Early Christian Ireland was not unpopulous although Niall may have improved the odds in his favour by killing or driving out a proportion of other males - including, if he is to be seen as the sole source for this marker, *all* his contemporary male relatives who, whether known to him or not, shared the same common male ancestry.I find this a difficult hypothesis to accept.ISTM far more likely that the common ancestor would have lived at a time when the population went through some sort of bottleneck; this is the explanation that population geneticists normally give for such an anomaly.From a palaeoecological point of view I think there are candidate periods somewhat earlier than the 5th century.
So how would I evaluate your other half's probability of being related to Niall?First of all, having discarded pro tem the TCD hegemony arguments, I need to evaluate the probability that Niall himself carried this marker.
There are the traditional associations which you quote of linking various surnames with descent from Niall.Certainly O'Neil is the name of a number of prestigious families in N. Ireland.It would be interesting to know if any of these have been tested and what the results are.However I don't think that genealogists who take a critical approach to traditional genealogy are satisfied that there is any reliable line from Niall to the modern day.Is there any historical evidence, in fact, to indicate that any modern surname's association with Niall points to anything closer a retainer of Niall or of one of his early descendants or even a retainer of a descendant of one of the earlier retainers?Above all we should be wary of using traditional genealogy to prop up DNA arguments which themselves are being used to attempt to verify the traditional genealogy and being sucked into circular arguments.
Going back to first principles, if we took the figure of a shade over 1 in 5 as a guide we would have to estimate the probability as being only this same 1 in 5 for Niall.This is, in fact, a pessimistic estimate.Demographic changes in the last half-millennium have reduced the proportion of any Early Christian marker.If you want to research it I'm sure you could find estimates for the proportion of plantation settlers in the relevant counties, the relative survival rates of the two populations in the subsequent ethnic/religious upheavals and in the 1840s famine which could give you some idea of the dilution.You would, however, have to allow for the fact that some of the planters would have Irish ancestry via Scotland.For practical purposes we can simply take the 1 in 5 as a minimum figure and say that the chances of your other half's sharing a common male ancestor with Niall is somewhat greater than 1 in 5 (note also that greater than 1 in 5 means that the hegemony hypothesis has to work even harder).And "sharing a common male ancestor" is, I think, as good as it gets.