Thanks for this link.It makes interesting reading.It also prompted me to re-read my copy of Mallory & McNeill1
It's quite clear that the research is written in terms of the U? Neill (who are well attested historically), not of any specified individual.In fact, what they have to say of Niall is "The ultimate origin of this dynasty is attributed to the conquering sons of the eponymous and possibly mythological 5th century warlord, Niall of the Nine Hostages" and "Figures such as Niall of the Nine Hostages reside at the cusp of mythology and history".It seems clear to me that they are specifically avoiding asserting the historicity of Niall.This approach is very different to saying that all bearers of this particular marker are descendants of Niall.Of course the material you included in your original post doesn't actually quite say they are - it only says they "may" be.
You may recall that I objected to the fact that the claim either assumed that the marker originated with Niall (a circumstance which would be so unlikely as to require separate proof) or that the descendants of his male relations carrying the same marker were being ignored.This objection does not arise when we are dealing with a kin group.
As a side comment on the Ui Neill it's known that they were divided up into northern and southern branches.This seems to be reflected in the distribution map in the paper.It appears that by mid first millennium BC the Ui Neill had become sufficiently numerous to have split into their two branches and, according to a map in Mallory & McNeill, migrated to their eventual homes from a place of origin well to the SE.This suggests to me a much earlier date of origin than the projected date of Niall.
Of course given that we seem to be dealing with a closely related group we need to consider its origins.The authors have this to say: "The time to the most-recent common ancestor (TMRCA) of this lineage was estimated ... At 1,730 (SD 670) years ago, it is at least consistent with an early medieval time frame."The SD (standard deviation) of such an estimate is an indication of the degree of precision.670 years is very wide (at least in comparison with such techniques as carbon dating) and from their comments it seems that the authors are disappointed with it; maybe they'd hoped to pin the TMRCA down to the C5th AD.In fact this result seems to me very exciting as it brings the Early Iron Age into scope.The palaeoecological evidence points to a break in human activity between the Later Bronze Age and Early Iron Age across quite a large swathe of N Ireland.Mallory & McNeill give a summary of this evidence.They also draw the same conclusion on archaeological evidence.As I've already said a small group of closely related peopleliving at this time would be very strongly represented in the later population without assuming that they fathered a disproportion number of children.The dating given in the paper is consistent with this hypothesis also.Finally, although we're getting well OT here, we're starting to run into questions such as the arrival of the Irish in Ireland, this being one of the suggested periods.
1 The Archaeology of Ulster, Mallory JP and McNeill TE, Institute of Irish Studies, Queens University Belfast 1991.Well worth reading if you can get hold of a copy.