If you plan on visiting just one megalithic site on your travels ... make sure it is Newgrange!
Purists may gasp now. Mainly because the almost blinding façade of quartz stone is a reconstruction. And not an undisputed one to boot - the archaeologists found the stones around the perimeter of the actual tomb and simply fit them into their concept of the "real" Newgrange. And, just by the way, we are not 100% sure whether Newgrange was intended to be a tomb at all. As you can see, Newgrange is surrounded by controversy.
And at the same time claimed by every stakeholder in all things prehistoric. Those believing in ancient astronauts cite Newgrange as an important area. Those searching for Celtic culture copy its pre-Celtic rock art faithfully. Those looking for "hidden knowledge" or earth mysteries find it in any or all carvings, alignments or measurements of the site.
I simply think that Newgrange is awe-inspiring. Situated on a hillside near the banks of the River Boyne, white as pure snow, with a chamber whose roof weighs more than several tanks. And which has survived millennia without mortar, held up by gravity alone.
And as we really do not know much about Newgrange, any reconstruction or theory seems to be valid to a point. Even the purists' view is just another unproven theory.