The dictionary account you refer to was most likely based on an older
printed history of Ireland. Recent histories are much more skeptical.
Niall is not a historical figure per se, but has assumed importance as
the common ancestor (and the source of the name of) the Ui Neill, the
politically dominent sept in Ireland at the dawn of recorded history.
However, there is nothing that can be said for certain regarding him.
What can be said is that he PROBABLY existed, and was probably the
ancestor of at least some (though hardly all) of the Irish families
which claimed descent from him. If he did exist, then he would almost
certainly have died in the middle of the fifth century, and not in 405
as the pseudohistorians gave it. No details of his life can be
discerned, as it is impossible to distinguish genuine tradition from
the political propoganda of the Ui Neill, who retroactively converted
Niall and his mythical ancestors into powerful Irish kings, in order
to enhance their political position by claiming that their family had
ruled Ireland from time immemorial.
Stewart Baldwin