I don’t accept that it’s a certainty. And the fact that it makes a good story is all the more reason to be sceptical. All we know for certain about the MRCA (Y) is that he had at least two sons. Through random chance, among all the people who had sons at the time, his happened to have sons in turn, and so on in an unbroken line down the generations to the present day. King Niall may have had eight sons, which puts him in an advantageous position to be a candidate for MRCA, but many other people from all walks of life (whose names are lost to history) also had eight sons.
They are both useful. You have a maternal lineage, through which the mDNA is transmitted, and a paternal lineage, through which the y-chromosome is transmitted to your father. Each extends in a single unbroken line, with one representative in each generation, back through history to the beginning of our species and beyond.
Taking any group of people, it is possible in principle to identify their most recent common ancestor (MRCA), their most recent common matrilineal ancestor and their most recent common male-line ancestor. These will in general be three different people living at different times. Of the three, the most recent will be the MRCA.
Me, but I haven’t the foggiest idea where my family (maternal) was before the 1790s. I really wish I did, though. I think it would be awesome to trace your family back hundreds of years, even if it’s only one branch.