Black Fort of Aran Coast

This is Inis Mor, or traditionally, just Aran,

the biggest of the three Aran Islands.

The people only live on the north-east side of the island,

their homes stretching like a long street from end to end.

To the south-west lies an expanse of bare rock,

punctuated here and there by a resilient blade of grass.

And yet people DID once live here, on the other side of the island.

Perched on top of dramatic 300ft cliffs is Dun Aonghusa,

acknowledged to be one of the finest prehistoric monuments in Western Europe.

But not for me the well-worn tourist trail.

No, I'm off in search of another ancient fort on a cliff edge.

Dun Duchathair...the Black Fort.

This landscape of booming ocean and jagged rock,

it all suggests power and strength and nothing says it more clearly than this massive curving wall.

The Victorians rebuilt this thing but they were working from a known footprint,

and they represented the known scale of the thing in their own hands.

It looks like the work of giants but it's not, it's the work of people

thousands of years ago and the scale of it takes your breath away.

But why go to all this Herculean effort

to cut off the end of this barren promontory?

I've come here to meet a man who can explain what's going on.

For some years, geologist Michael Williams has also been trying

to solve the conundrum of the strange location of the Black Fort.

Did the habitation of the interior actually reach all the way to the cliff edge?

I think it certainly did because here we are in the central part

of the promontory

and a very exposed part of the promontory and yet here we have a constructed,

human-constructed wall, you can see it quite clearly, the leading edge here.

You can see courses of it there. Exactly.

But the habitation is not restricted simply to the inside wall of that so-called promontory fort.

It actually exists here, out in the exposed promontory.

But what about these huge boulders, they're not part of the fort?

People might find this hard to believe, but these are actually washed up

on top of this promontory 65 feet above sea level,

by the giant waves that affect these islands on a regular basis.

So why would you go to all that effort to block off this end

of the promontory when it's being pounded by these things?

The mistake we're making is looking at this fort in the context of today,

whereas we should be looking at it in the context of 2,500 years ago when it may have been built in the first place.

If we make a sort of rough estimate of the rates of erosion that produce this kind of debris

and say that it comes to about 0.4 metres a year.

Extrapolate that for 2,500 years, let's say, and we're looking at land

extending from here, a kilometre out into the Atlantic Ocean.

So what's now known as Inis Mor, The Big Island,

2,500 years ago, was a heck of a lot bigger.

So we're not looking at a promontory fort at all.

We're talking about the remnants of a massive circular fort.

So we should be enclosed now by the continuation of that massive wall.

But what makes absolute sense is that any seas that can fling

five tonne boulders far inland could eat a hilltop fort for breakfast.

This destructive energy is borne out as we pass over Inis Meain and Inis Oirr,

the other two Aran Islands.

It's as though they've been cut in half with a blunt saw.

MUSIC: "Theme Tune to Father Ted" by The Divine Comedy

At the most easterly end of Inis Oirr, the rusting hulk of a cargo ship, the Plassey.

But this ship is special, immortalised in the title sequence of the comedy series, Father Ted.

But, no time to dawdle. We too must...

IN IRISH ACCENT: "Go on, go on, go on."