Ian Adamson's book is used by the British Unionists as they see it as a 'justification' for their existance in Northern Ireland. Adamson himself is a Unionist.
Adamson claims that one of the 'original' tribes inhabiting Ireland particularly Ulster were a tribe known as the Criuthni....in Scotland they were known as Picts.....in Wales and England the Pretani....hence the word Britain which came from it. He claims they were one of the first peoples to inhabit Ireland coming across a landbridge from Scotland.
He claims the Cruithni (the people of the Stone Age in Ireland) were driven out of Ireland to the extremes of Scotland by the Gaels or Celts (who brought the advent of the Iron Age and Bronze Age in Ireland).
Some Northern British Unionists today claim that the conquering and plantations of Ireland of the 1600s and after by the Ulster Scots was an act of them rightfully retaking their land back.... hence you see the likes of the mural in Loyalist areas proclaiming 'Cuchulainn .....ancient defender of Ulster against Irish attacks ....2000 BC'.
However, all bonkers really how folklore, mythology and religion can get out of hand...for we here today in the rest and all of Ireland can be just as much part Cruithni, Gael/Celt, Norman, Viking, British or Irish as anyone else.....and placenames and surnames are the evidence.
It was probably the Cruithni that built Newgrange and the likes in Knowth and Dowth....great emblems of our Stone Age.
(the surname 'Rooney' is supposedly of Cruithni origins )