Sorry to be a wet blanket but even if my McKee BiL were to be tested and found to match it'd take more than that to persuade me that my children were descended form Niall.
Firstly it assumes, and I don't see anything stated here to back this up, that Niall actually had this marker.If, however, we accept this assumption what we have is evidence of kinship between any modern carrier of the marker and Niall.
There are only two ways in which one could take this as indicative of descent.
One is that the marker originated in Niall by mutation.
The other is that none of Niall's contemporary male kin left any male line descendants.
Neither seems to me to be susceptible of proof and neither seems particularly probable.
I would suggest an alternative explanation: this was a common marker in the C5th as it is now and that Niall was simply one of the many carriers.This is no more susceptible of proof than the others.It simply seems more probable.
Again, sorry to be a wet blanket but kinship does not imply descent.