Ancient Ireland

It wasn't so long ago that Homer's Iliad was believed to be no more than a collection of myths and legends.  But in 1870 Schliemann, by following The Iliad, discovered Troy.  Is it possible that The Celtic Legend of Oisin in the Land of Youth and the High Deeds of Finn from ancient Ireland are, just as Homer's Iliad, not myth but based on fact?  The observatory at Newgrange in Ireland suggests that its creators had an advanced culture and must have been studying mathematics, astronomy and time to a high level around 5,500 years ago, well over a thousand years before either Greece or Egypt had started their rise to civilisation.  In 'The promise Tree' a female investigator starts to pick up the pieces.  She is following the intangible, the invisible, the romantic in her everyday life.  But gradually what she is following begins to have form.  Were the Irish of Newgrange, balanced as they were on the edge of western Europe, all that was left of what was possibly the earliest advanced culture of our civilised world?