As a direct descendant of this line, allow me to give my input :
Disclaimer : I do not know anything about genealogy. I am not a history buff. I am not a big fan of irish mythology and have never studied it. I have always been a christian and i have always been up in the air about if the bible is literal or figurative.
I am also a very driven and observant person who uses logic and reason to come to correct conclusions.
Also, I had never heard of Niall, and don't personally care if people say he's fake. There is no biased.
That being said, Niall shows as my 45th Great Grandfather. Just not correctly. I have found the direct paternal link throughout my family to the first man.
The direct paternal link doesn't show on geni, because they won't acknowledge that Marcus Robertson, my Great great great Grandfather, was born to Chief Justice George Robertson or one of his sons, if you fix that chain in the link then it shows from Niall to me descendant by descendant, through the line of Duncan I.
But the indirect way, going through my great great great grandmother's side still shows, and ends up at the same result. I have no personal biased in wanting it right, it already shows the heritage, I just want it to be correct.
All that being said, I traced my lineage, father by father to Niall, therefore, Niall exist. He's my Grandpa.
Now, on to the rest of the story.
Even though I didn't know anything at all about Irish history or "mythology", when I kept tracing the line of my ancestors, using ancestry.com and logic, it just keeps going all the way back, and gives a recorded timeline, generation by generation of the history of Ireland and then back into the days of the bible!
Now, because I have connected to geni, a lot of major kings throughout history show up as my grandfathers.
I believe that the High Kings of Ireland where real people. My strongest evidence is this :
To create a timeline of any type of Royal Family going backwards would be impossible.
However, that's what the Royal pedigree is, a timeline of exactly who was born, even keeping track of the cousins through the Earl system.
They couldn't have made it up, because it includes too many relatives, and the relatives it includes are the people who are recognized throughout the history of this area.
You can't just read the lineage they provided through history , and accept what you want out of it.
Either you throw out the whole book and say "We don't believe any of these people, anything over 200 years is a no go" , or you believe everything they say.
That's logical.
That being said, I highly highly highly believe this page should be merged with the "fiction" page, so that the lineage continues to show.