NARRATOR: Ireland.
This lush, green island in the North Atlantic
has a history of human activity
dating back more than 33,000 years.
Numerous cultures have called Ireland home,
from the early Beaker People, to the Celts and the Gaels.
And the island has a rich tradition of folklore
that has spread throughout the world.
But perhaps most notable is that Ireland
boasts some of the oldest structures on the planet.
Massive earthen mounds,
many of which date back more than 5,000 years.
TOK THOMPSON: These mounds are constructed. They're not natural hills.
These are megalithic constructions
involving, in the larger sites, an enormous amount
of man hours and labor.
These would have been, when they were made,
just very, very impressive sites.
CHILDRESS: These mounds were an ancient earth work
that had a tunnel going through it,
with stones on either side, and stones on top
and then covered with earth.
THOMPSON: Ireland has a tremendous amount of these mounds.
These are some of mankind's oldest,
large structures in the world.
And they anchor all the land together.
Usually if you visit one on top of a hill, you go up,
you find one, and you can do a line of sight to the next one.
There was almost a network, almost across
the entire island of these sites.
NARRATOR: The oldest known writing in Ireland
dates to the sixth century AD,
more than 3,000 years after the ancient mounds
are believed to have been built.
With no written record about the mounds' construction,
archaeologists and historians can only speculate
as to who made them, and for what purpose.
THOMPSON: These things were built to go inside,
where presumably very, very important rituals took place.
We don't know, but much later on,
these sites would have been used for gatherings,
for rituals, and sort of keeping the society together.
These were enormously long-lived sites of cultural importance.
It was enormously long-lived societies
that used these as focal points.
NARRATOR: 30 miles north of Dublin in County Meath
is Ireland's largest and oldest mound, called Newgrange.
The ancient structure consists of over
200,000 tons of stone,
and has a passageway leading to a central chamber,
in which human bone fragments have been found.
But archaeologists say Newgrange also has
an undeniable connection to the cosmos.
Spiral petroglyphs thought to symbolize the universe
are found all around it, and the passageway and chamber
form a cruciform shape that researchers believe
is based on the constellation Cygnus.
At the time Newgrange was built,
it was presumed to be the largest structure
that mankind had ever built in the world.
Now, one of the most interesting things about Newgrange
is its celestial alignment.
This is still very, very popular.
If you go at the winter solstice,
you have this sun dagger
that it cuts tremendous distance,
all the way in to brighten up the inner chamber.
NARRATOR: Newgrange is unique among Ireland's mounds
in its sophistication, and was clearly designed
for ritualistic activity.
But according to ancient Irish texts,
the mounds serve a more profound,
and mysterious purpose, as the dwelling places
of otherworldly beings
traditionally called the "sidhe."
JENNY BUTLER: The word "sidhe," it can mean the mounds,
and also can mean the otherworld people,
the people of the mounds.
In Irish tradition, it's bad luck
to use the word "sidhe" to directly refer
to these otherworld people.
So, there are many different names.
We have the English word fairy folk, also elves.
NARRATOR: Stories of fairies, elves, and even leprechauns,
are rooted in the ancient folklore
of what the Irish called the sidhe.
But for many Irish people,
the sidhe are not merely mythological figures.
MARTY MULLIGAN: Fairies and leprechauns and all that sort of stuff, like,
too many people kind of laugh and joke about it at times.
But it's knowledge that had been passed on
for thousands and thousands of years.
We have the terrestrial world,
and the sub-terrestrial world, the human world,
and the superhuman or other world.
These worlds exist in parallel.
Occasionally, however, there is communication
between the two worlds.
NARRATOR: The mounds are often referred to as fairy forts,
as are stone circles and tree clusters
that are considered the property of the sidhe.
This strong cultural belief that these features
are the domain of otherworldly beings
has kept them intact for thousands of years.
And the Irish say that when they are disturbed,
bad luck always follows.
MULLIGAN: Sean Quinn was the wealthiest man in Ireland.
He lost everything because he moved a fairy fort.
He damaged a fairy fort,
and he moved it to another place.
DeLorean car manufacturers, they came to Belfast
and were making the DeLorean car up in Northern Ireland.
And they built the factory on the site of a fairy fort.
And it all came crashing down.
The local people would say,
"Well, you know, what do you expect?"
Ask any Irish person, "Would you damage a fairy fort?
What would happen if you damaged a fairy tree?"
If you gave me ten million bucks
and a Cadillac and a helicopter
and a beautiful house on Sunset Boulevard,
and said all I had to do was chop down a fairy tree,
I'd be handing that money straight back to you.
I wouldn't do it, no one I know would do it either.
So, do we believe in the fairies in Ireland?
We do.